问题

Some people view the findings with caution, noting that a cause－and－effect relationship be

Some people view the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remains______.

A．to be shown

B．to have shown

C．to have been shown

D．being shown

请帮忙给出正确答案和分析，谢谢！

参考答案

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