问题补充：
The tourists didn’t stop to rest at each station because it them down.
A. had slowed
B. would slow
C. should have slowed
D. would have slowed
答案：
A. had slowed
B. would slow
C. should have slowed
(答案→)D. would have slowed
解析：would have slowed 是虚拟语气，其后省略if they had stopped at each station
