Question

  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

Answer 1

C

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。
考点：考查名词性从句。