repeatedly, the problem still appeared in the beginners’ homework.
A. Explaining
B. Having explained
C. Having been explained
D. To be explained
网友回答
A. Explaining
B. Having explained
(答案→)C. Having been explained
D. To be explained
解析：考察分词作状语。本题中的动词explain与后面句子主语the problem之间构成被动关系，故使用过去分词done表示被动，在很多情况下done=having done；前者更多强调被动，后者更多强调已经完成。句意：已经被重复地多次解释，这个问题还是出现在初学者的家庭作业里。故C正确。AB项没有被动的形式，D项中的不定式表示的是将来时。