10.Some people viewed the findings with caution,noting that a cause-and-effect relationship...
Some people viewed the findings with caution,noting that a cause-and-effect relationship between passive somking and cancer remains____.
A.to have shown
B.to be shown
C.to have been shown
D.being shown
为什么选B,我基础不好,请讲详细点,分析考点及解题思路