is known to us is that the Olympic Games will be held in Rio de Janeiro Brazil.A. It

问题补充：

is known to us is that the Olympic Games will be held in Rio de Janeiro, Brazil.

A. It

B. Which

C. As

D. What

答案：

A. It

B. Which

C. As

(答案→)D. What

解析：考查主语从句。本题中的what引导主语从句what is known to us,并在句中做主语。It不能引导主语从句，which指特定范围之内的某一个；as不能引导名词性从句。What引导名词性从句，在句中做主语，宾语，表语等成分。句意：众所周知的是奥运会将在巴西的里约热内卢举行。故D正确。