It is that he has been addicted to the drugs and has difficulty quitting it.A. obviouslyB.

问题补充：

It is that he has been addicted to the drugs and has difficulty quitting it.

A. obviously

B. apparently

C. possibly

D. likely

答案：

A. obviously

B. apparently

C. possibly

(答案→)D. likely

解析：考察形容词作表语。形容词可用在be动词后作表语。副词一般用在实意动词后做状语，句中is后应用形容词作表语，ABC选项都是副词，故只能选D，很可能的，既能做副词，又能做形容词。