the students spent as much time getting trained as they had studying.为什么用did 不用hadas they did studying 选的did 不是had

the students spent as much time getting trained as they had studying.为什么用did 不用had
as they did studying 选的did 不是had

那就解释为did代替spent.我以为,两者都可以,只是含义不同.用did,表明学习和训练同时发生；用had (spent),表明学习先于训练发生.