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

从事海上拖带的船由深水区进入浅水区，应：（）

A.拖缆缩短，降低拖速

B.拖缆缩短，增大拖速

C.拖缆加长，增大拖速

D.拖缆加长，降低拖速