单项选择题

A. keep planes from :flying B. bury sand on ocean floor……

Air pollution is caused by many things. Great clouds of smoke come from factories. Cars send smoke and fumes into the air. Burning waste fills the air with bits of dirt. Even burning leaves add smoke to the air. Sometimes, planes cannot find their landing spots through smoke. Smoke of all kinds is bad to breathe and can do great harm to the body.
Still, we cannot live without air. We must find a way to clean the air we have.
In time, we may have factories that are run by atomic energy. Our cars may run on smoke-free electric power. Scientists are also working on new ways to help the oil burned by cars from making fumes. Waste may be burned in the sand on ocean floors.
These changes might keep our air clean in the years to come. But until then, many scientists are looking for ways to make air cleaner now.
Scientists are now working on ways to ______.

A. keep planes from :flying
B. bury sand on ocean floors
C. stop air pollution
D. burn leaves without smoke

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单项选择题Finally it gave (Paragraph 5) means that ______.

A. Luckily the door was torn away in the end
B. At last the wrench went broken
C. The lock came open after all his efforts
D. The chance was lost at the last minute

单项选择题The story does not say so, but it makes you think that ______.

A. stopping air pollution is hard
B. dirty air is blown away by the wind
C. planes cannot fly through smoke
D. it is impossible to make dirty air cleaner