Some people viewed the findings with caution, noting that a cause-effect relationship between smoking and cancer remains ______.

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问题 Some people viewed the findings with caution, noting that a cause-effect relationship between smoking and cancer remains ______.

选项 A、to be shown
B、to have shown
C、to have been shown
D、being shown

答案A

解析 结合选项的意思可以看出该题的大概意思,即:“有些人对这些结果持谨慎态度,认为吸烟与癌症的因果关系有待证实。”关系是被证实,属于被动,因此将主动的B先排除。有待证实是将来的事,动词的形式应是不定式,因此D也可以排除。A和C的区别在于时态,根据题意,显然A为正确答案。
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