有一题请教各位高手

The fear of rabines 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.

请问各位高手，两个答案最大的区别在哪？正确答案是（B）