Scientists perform careful experiments and do wide research because______.

单项选择题While it is not directly stated, the article suggests that______.

单项选择题For as long as humans have raised crops as a source of food and other products, insects have damaged them. Between 1870 and 1880, locusts ate millions of dollars' worth of crops in the Mississippi Valley. Today in the United States the cotton boll weevil damages about 300 million dollars' worth of crops each year. Additional millions are lost each year to the appetites of other plant-eating insects. Some of these are corn borers, gypsy moths, potato beetles, and Japanese beetles. In modern times, many powerful insecticides(杀虫剂) have been used in an attempt to destroy insects that damage crops and trees. Some kinds of insecticides, when carefully used, have worked well. Yet the same insecticides have caused some unexpected problems. In one large area, an insecticide was used against Japanese beetles, which eat almost any kind of flower or leaf. Shortly afterward, the number of corn borers almost doubled. As intended, the insecticide had killed many Japanese beetles. But it had killed many of the insect enemies of the corn borer as well. In another case, an insecticide was used in Louisiana to kill the troublesome fire ant. The insecticide did not kill many fire ants. It did kill several small animals. It also killed some insect enemies of the sugarcane borer, a much more destructive pest than the fire ants. As a result, the number of sugarcane borers increased and severely damaged the sugarcane crop. To be sure that one insect pest will not be traded for another when an insecticide is used, scientists must perform careful experiments and do wide research. The experiments and research provide knowledge of the possible hazards an insecticide may bring to plant and animal communities. Without such knowledge, we have found that nature sometimes responds to insecticides in unexpected ways. An insecticide was used in Louisiana to kill the troublesome______.

单项选择题You stare at waterfall for a minute or two, and then shift your gaze to its surroundings. What you now see appears to drift upward. These optical illusions occur because the brain is constantly matching its model of reality to signals from the body’s sensors and interpreting what must be happening—that your brain must have moved, not the other; that downward motions is now normal, so a change from it must now be perceived as upward motion. The sensors that make this magic are of two kinds. Each eye contains about 120 million rods, which provide somewhat blurry black and white vision. These are the windows of night vision; once adapted to the dark, they can detect a candle burning ten miles away. Color vision in each eye comes from six to seven million structures called cones. Under ideal conditions, every cone can “see” the entire rainbow spectrum of visible colors, but one type of cone is most sensitive to red, another to green, a third to blue. Rods and cones send their messages pulsing an average 20 to 25 times per second along the optic nerve. We see an image for a fraction of a second longer than it actually appears. In movies, reels of still photographs are projected onto screens at 24 frames per second, tricking our eyes into seeing a continuous moving picture. Like apparent motion, color vision is also subject to unusual effects. When day gives way to night, twilight brings what the poet T.S. Eliot called “the violet hour.” A light levels fall, the rods become progressively less responsive. Rods are most sensitive to the shorter wavelengths of blue and green, and they impart a strange vividness to the garden’s blue flowers. However, look at a white shirt during the reddish light of sunset, and you’ll still see it in its “true” color—white, not red. Our eyes are constantly comparing an object against its surroundings. They therefore observe the effect of a shift in the color of illuminating on both, and adjust accordingly. The eyes can distinguish several million graduations of light and shade of color. Each waking second they flash tens of millions of pieces of information to the brain, which weaves them incessantly into a picture of the world around us. Yet all this is done at the back of each eye by a fabric of sensors, called the retina, about as wide and as thick as a postage stamp. As the Renaissance inventor and artist Leonardo da Vinci wrote in wonder, “Who would believe that so small a space could contain the images of all the universe?” Visual illusions often take place when the image of reality is ___.

单项选择题According to the passage what people generally consider religion to be?

单项选择题What does the word “observance” probably convey in Para. 1?

单项选择题The best title which expresses the idea of the passage is_______.

单项选择题In private conversation, women speak

单项选择题The theme of this article is _______.

单项选择题Which of these words is nearest in meaning to the words 'are divided' in Paragraph

单项选择题Every day 25 milion U. S. children ride school buses. The safety record for these buses is much better than for passenger cars; but nevertheless, about 10 children are killed each year riding on large school buses, and nearly four times that number are killed outside buses in the loading zones. By and large, however, the nation's school children are transported to and from school safely. Even though the number of school bus casualties(死亡人数) is not large, the safety of children is always of intense public concern. While everyone wants to see children transported safely, people are divided about what needs to be done—particularly whether seat belts should be mandatory (强制性的). Supporters of seat belts on school buses argue that seat belts are necessary not only to reduce death and injury, but also to teach children lessons about the importance of using them routinely in any moving vehicle. A side benefit, they point out, is that seat belts help keep children in their seats, away from the bus driver. Opponents of seat belt installation suggest that children are already well protected by the school buses that follow the National Highway Traffic Safety Administration's (NHTSA) safety requirement set in 1977. They also believe that many children won't wear seat belts anyway, and that they may damage the belts or use them as weapons to hurt other children. A new Research Council report on school bus safety suggests that there are alternate safety devices and procedures that may be more effective and less expensive. For example, the study committee suggested that raising seat backs four inches may have the same safety effectiveness as seat belts. The report sponsored by the Department of Transportation at the request of Congress, reviews seat belts extensively while taking a broader look at safety in and around school buses. Each year, children killed outside buses in the loading zones are about_______.