问题
选择题
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
答案
B
题目分析：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。