一道有关语法的题，请大师指点。

这道题：

The fear of rabies is well founded; few people are known to recovered 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.

请问：1.为什么正确选项B里用to have recovered而不是 to recover？

          2.after后面能否加名词？几乎所有after加名词的选项都错了

          3.once对时态有什么影响？

1.完成时强调对现在的影响--fear.

2.once比after好，贴切，更达意，--一旦发现。。

选最佳的

啊，谢了