some teenagers don't realize is difficult life can be after they get addicted to drugs.

问题补充：

some teenagers don realize is difficult life can be after they get addicted to drugs.

A. What; what a

B. That; how

C. What; how

D. That; what

答案：

A. What; what a

B. That; how

(答案→)C. What; how

D. That; what

解析：C 考察主语介词和表语从句。第一空使用what引导主语从句，what在主语从句作为realize的宾语，第二空选用how引导表语从句，how修饰形容词difficult。