求教，新东方语法练习题1第10题，请各位大虾不吝赐教

nicklover

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

我不明白a和b有什么具体的区别，为什么一定要选b，而不能选a。  谢谢~~~~

nicklover

有没有人来帮帮我呀~~~~

Cathelin

由第一话可知，本题目需要用完成时态，因为只有这样才能说明第一句话，语法题目多做做就有语感了，加油！