单项选择题

Which of these statements is TRUE of system building？（）

A.It employs more men.
B.It is difficult and dangerous.
C.It can save both time and money.
D.It means less mechanization.

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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

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