If you are weak a subject and asking a teacher for advice can help.A. at; working hardB

问题补充：

If you are weak a subject, and asking a teacher for advice can help.

A. at; working hard

B. in; work hard

C. at; work hard

D. in; working hard

答案：

A. at; working hard

B. in; work hard

C. at; work hard

(答案→)D. in; working hard

解析：句意：如果你在某一科不行，努力学习并向老师请求帮助可能有用。短语be weak in在……方面不行，be good at擅长……，现在分词作主语表示抽象的动作，结合语境可知选D。