__  carefully if any change occurs when doing experiments in the lab..A.ObserveB

__  carefully if any change occurs when doing experiments in the lab..A.ObserveB

题型:不详难度:来源:
__  carefully if any change occurs when doing experiments in the lab..
A.ObserveB.To observe
C.ObservedD.Observing

答案
A
解析

试题分析:本题考查非谓语动词。考查方式为识别是否适用非谓语动词。根据上下文可知,后面提供了两个从句,一个为条件状语从句、另一个为时间状语从句的省略句,由此可知,在某条件下和某时间,建议某人做某事。因此,应该选择动词原形,引起的是祈使句。所以,本题选A。
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