单项选择题

While it is not directly stated，the article suggests that（）.

A.insecticides are not dangerous to any small animals
B.insecticides do not always accomplish their purposes
C.insecticides are no longer being used to kill insects
D.insecticides do no harm to people

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单项选择题

Scientists perform careful experiments and do wide research because（）.

A.they must learn to destroy all the insects that we need
B.they must be sure one insect pest is not traded for another
C.research keeps them from inventing new insecticide for the crops
D.research helps them find a way to kill all insects

单项选择题

The usual fixed length in the modular system is（）.

A.twenty centimeters
B.fifty centimeters
C.ten centimeters