题目内容：
Some people viewed the findings with caution, noting that a cause relationship between passive smoking and cancer remains （）.
A.to be shown
B.to have shown
C.to have been shown
D.being shown
参考答案：
选择题：Some people viewed the findings with caution, noting that a
- 题目分类：中国大学MOOC慕课
- 题目类型：选择题
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