—What’s the matter with you? You look so worried.-- I don’t know the problem.A. what to s

问题补充：

—What’s the matter with you? You look so worried.

-- I don’t know the problem.

A. what to solve

B. how to solve

C. to solve what

D. how can I

答案：

A. what to solve

(答案→)B. how to solve

C. to solve what

D. how can I

解析：根据上文句意，你怎么了？你看起来这么焦虑。——我不知道如何解决这个问题。What意思是什么，指具体事物。How是怎么怎样的意思，指方式。根据题意可知这是说的是解决问题的方式，选项D缺少谓语动词，故选B。