for和of用法的区别
1.It's necessary ______us to master a language.
2.It's unwise ______them to refuse our help.
3.It is very important _____us to study some rules.
4.It is unpracticable ______us to finish such a difficult task
in such short time.
5.It's wrong _____a person to think only of himself.
哪些用of哪些用for,请说明原因,我老分不清楚.