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
网友回答
A. As
B. It
(答案→)C. What
D. Which
解析：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。