Even if there is no scientific proof secondhand smoke causes cancer, there is no reason why nonsmokers should be forced to take this risk.
A. which
B. that
C. when
D. where
网友回答
A. which
(答案→)B. that
C. when
D. where
解析：考查同位语从句：句意：即使没有科学证明二手烟能致癌，但也没有理由非吸烟者要去冒这个险。That引导的是同位语从句，说明proof的内容，that不做成分，只起到连接的作用。选B。