Mr. Bill liked shooting very much but he was never good at it. A month ago some of his friends visited him in his house and saw a new target(靶子)which Bill had put several days before in his garden.
His friends went nearer and looked at this beautiful target. There was a hole right in the middle of the target. When they asked who had shot the target, Bill said he haD.They all laughed and said,' How far away were you, Bill? Two feet?' But Bill said he was fifty yards(码)away.
Then Mr. Bill's wife explained about the hole in the middlE.She said,' Bill went to a shop and bought a very big piece of wooD.He brought it home in a car, put it in the garden and shot at it from fifty yards away. Then he drew a target round the hole and cut the wooD.'
One day some of Bill's friends saw ______ in his garden.
A.a very big piece of wood
B.a very nice target
C.a hole
D.a very beautiful picture

不定项选择‌厌氧处理法要求的N、P营养物质高于好氧处理法。‎

未分类题A new kind of radar has been developed for spaceship travelers. A working laboratory model of a new system of radar that makes use of a beam of light is said to be ten thousand times more accurate than the best comparable system of radar that uses microwaves.The model has shown that this radar system, known as laser-dopple radar, can measure with absolute precision speeds varying from spaceship orbital injection(进入)velocities(速度)of five miles per second down to virtual stops-speeds of less than one-thousandth of an inch per seconD.According to the scientists who are developing this system, such fine measures of velocity are of prime importance in space missions. In a rendezvous(对接) between two spaceships, or in a landing approach by a vehicle onto an orbiting space station, a bump could rip open a ship's skin, or a nudge could knock the station out of its orbit.The light-beam radar, which operates at a frequency of trillions of cycles(百万兆) per second, could easily detect and measure the movement of a vehicle edging up to a satellite space station. A control system using so precise a signal as this would allow a huge vehicle to dock at a space station as lightly as a feather.Laser-dopple radar ______ .A.measures the movement of a spaceship by means of light beamB.makes use of microwavesC.makes use of sound wavesD.both A and B

单项选择题The stars twinkled in the transparent clarity of the sky. （ )

单项选择题1.第一台计算机是哪一年发明的（） A、1950年 B、1956年 C、1946年 D、1949年

未分类题The difference between a liquid and a gas is obvious under the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fill it to the level of a free surfacE.A gas forms no free surface but tends to diffuse(扩散)throughout the space available; it must therefore be kept in a closed container or held by a gravitational field, as in the case of a planet's atmospherE.The distinction was a prominent feature of early theories describing the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be' dissolved' in a vapor without losing its identity, and another theory that held the two phases are made up of different kinds of molecules. liquids and gases. The theories now prevailing take a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both flow readily. They are fluids.The fundamental similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. Suppose a closed container partially filled with a liquid is heateD.The liquid expands, or in other words becomes less dense; some of it evaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and press at which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be distinguished; there is a single, undifferentiated fluid phase of uniform. density.Which of the following would be the most appropriate title for the passage?A.The Properities of Gases and Liquids.B.What Fluids and Gases Are Made oF.C.New Uses for the Gas and the LiquiD.D.New Theories about Fluids and Gases.