[求助]大全，557/309

557.One of every two new businesses fail within two years.

(A) fail

(B) fails

(C) should fail

(D) may have failed

(E) has failed

在看到within 这种表示一段时间的短语，为何不选E而选B呢？

309.For some birds the sense of smell appears to play a role in navigation, since pigeons with surgically removed olfactory nerves were found to have increased difficulties in homing.

(A) were found to have increased difficulties

(B) have been found to have increased difficulty

(C) were found to have increasing difficulty

(D) had been found to have increased difficulties

(E) have been found to have increasing difficulties

这道题也很奇怪，选了B而不是A，没有时间副词为何要用完成时呢？？？

557，1,within past 2 years.   2,完成时没有 规律 的意义。

309，http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=39455