问题
选择题
______ is known to us is that tobacco contains nicotine and other harmful products, so you should give up smoking.
A．As
B．It
C．What
D．Which
答案
答案：C
题目分析：考查主语从句：句意：我们都知道的是香烟含有尼古丁和其他有害的物质，所以你应该戒烟。第二个is是谓语，前面的是主语从句，主语从句缺少主语，用what连接主语从句，选C。