while walking in the street ,you have to be very careful to

while walking in the street ,you have to be very careful to
avoid__down by passing cars or trucks.a.to be knocked b.being knocked
答案为什么不是a?不应该是将来吗?虽然avoid+doing,可是前面还有while while 的主句不是要+done吗?
在什么时候while +done：doing+when?
都表示什么?
就像这道题while watching tv,___
a.the doorbell rang b.we heard the doorbell ring c.we heard
the doorbell rings d.we heard the doorbell rang
如果是B.C.D怎么分析?
为什么?