叙述过去经历 — 英语选择题

昨天下午四点，大雨开始下的时候，Ben 正在图书馆看书。下面哪个句子正确叙述了这段经历？A. When the rain started, Ben read books in the library. B. When the rain started, Ben was reading books in the library. C. While the rain started, Ben was reading books in the library. D. When the rain was starting, Ben read books in the library.