When questions in class one should answer them as clearly as possible.A. askingB. to ask

问题补充：

When questions in class, one should answer them as clearly as possible.

A. asking

B. to ask

C. to be asked

D. asked

答案：

A. asking

B. to ask

C. to be asked

(答案→)D. asked

解析：考查句子结构省略。在when引导的状语从句中，如果主语和主句的主语相同，而动词又是be动词的时候，主语与be动词往往被省略。此句省略了one be 完整的句子是when one is asked...,A表示进行时态，BC不定式表示将来，D在这里表示被动，句意是在课堂上被提问时，你回答问题要尽可能清晰。故选D.