找出错误项并在横线上改错
1.I will return the book back in two days.-----
a.will b.return c.back d.in the days
2.Todays we'll climb a mountain which is 800 meters over sea level.-----
a.climb b.isc.800 meters d.over
3.I don't know when will he come back .-----
a.don't knowb.whenc.will hed. come back
4.I think he isn't a very hard-working student.-----
a.think he isn'tb. ac.very d.hard-working