Some people viewed the findings with caution, noting that a cause-and-effect rel

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

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

答案 A

解析考查固定用法。remain to be done表示“有待于(被做)”，表示尚未做某事，故无需用被动语态的完成式，可排除B，选A。

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