大全-748, 补充-010,once可以接完成时吗?

10.   The fear of rabies is well founded: few people are known to recover from the disease after the appearance of the clinical symptoms.

(A) few people are known to recover from the disease after the appearance of the clinical symptoms

(B) few people are known to have recovered from the disease once the clinical symptoms have appeared

(C) there are few known people who have recovered from the disease once the clinical symptoms have appeared

(D) after the clinical symptoms appear, there are few known people who have recovered from the disease（B）

(E) recovery from the disease is known for only a few people after the clinical symptoms appear

觉得once不能接完成时呀,而且句意是"从..appear"那一刻起,怎么要用完成时呢? 所以我就排除B选A了.....要考试了反而晕忽忽的了,NN帮忙啊