Now people have come to realize harmful pollution is.A. that whatB. itC. that howD. how

问题补充：

Now people have come to realize harmful pollution is.

A. that what

B. it

C. that how

D. how

答案：

A. that what

B. it

C. that how

(答案→)D. how

解析：句意：现在人们逐渐的认识到污染多么的有害。在宾语从句中连接词不能同时应用，how用来修饰形容词的程度，故选D。