is known to all is that the Diaoyu Islands have belonged to China since ancient times.A. A

问题补充：

is known to all is that the Diaoyu Islands have belonged to China since ancient times.

A. As

B. It

C. What

D. That

答案：

A. As

B. It

(答案→)C. What

D. That

解析：句意：我们都知道钓鱼岛自从古代就属于中国。That在主语从句中不做成分，只起连接是作用；这里缺少主语从句的连词，并且连词在句中做主语，根据句意故选C。