They had wanted to help but___get there in time

问题描述:

They had wanted to help but___get there in time
A can"t
B couldn"t
为什么选B

couldn't 是 can't 的过去形式; 通过 wanted可以分析出这句话说的是过去的情况.
他们“不能”准时到达这个“不能”是过去的“不能”.