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：
试题分析:句意：我们都知道烟草里含有尼古丁和其他有害物质，所以你应该戒烟。分析结构可知第二个is前是主语从句，从句中缺少主语，which强调范围内的选择，而what强调内容，所以选C。