To observe the living habit of pandas, ________．A.the poor little creatures were isolated in the zooB.they were set free to the wildC.they walked deep into the mountainD.a long period of time was needed
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C解析当动词不定式用作状语时，要注意它的逻辑主语必须和主句的主语保持一致。在这句句子中，动词不定式to observe the living habit of pandas的逻辑主语为人，但是前两个选项的主语是熊猫，而选项D的主语是时间，所以只有选项C是正确的。