独立主格结构什么时候用完成时?
如这个题：
All flights ______ because of the snowstorm,many passengers could do nothing but take the train.
A）had been canceled B）have been canceled
C）were canceled D）having been canceled
因为风雪,所有航班都被取消,许多旅客别无它法,只得乘火车.(D)
All things ______,the planned trip will have to be called off.
A）considered B）be considered
C）considering D）having considered
考虑了方方面面之后,原来计划好的旅行不得不取消了.(A）
为什么前者要用完成时而后者不用?