单选题 ______ is known to us is that tobacco contains nicotine and other harmful products, so you should give up smoking.

【正确答案】 C

【答案解析】解析：考查主语从句。句意：众所周知，烟草中含有尼古丁和其他有害的产品，所以你应该戒烟。此处what引导主语从句，在从句中作主语。