锅炉水位应经常保持在正常水位线处，并允许在正常水位线上下()mm之内波动。
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Zimbabwe's government policies have actually helped neighbouring countries in one way.
A. 真
B. 假
C. NOT GIVEN
苯的爆炸极限(V/V%.)是()。
A. 8.0～12.0
B. 8.0～42.0
C. 1.2～1.9
D. 1.2～8.0
That fire ants prey on bollworms and budworms is mentioned to show that
A. pests often attack each other to maintain a balance of nature.
B. fire ants are not always harmful pests in given situations.
C. fire ants are stronger and fiercer than bollworms and budworms.
D. pests are either harmful or beneficial to people or plants.
The author's tone in talking about fire ants in the US seems to be
A. critical.
B. indifferent.
C. fearful.
D. objective.
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