spend on doing sth.指花费金钱做某事;
spend on sth. 指花费金钱.时间在某事上;
spend in doing sth.指花费时间做某事.
I spent what little time I had with my family.
我仅有的一点儿时间都和家人在一起度过了。
The money could be better spent on more urgent cases.
这笔钱用于较紧迫的事情也许会好些。
I spent most time on the first question.
我在第一个问题上花的时间最多。
