答案：
1.be recycled
2.about
3.between
1.本句含义“废弃的纸可以回收利用嘛?”
Can paper （that is threw away ）be recycled?
that is threw away是修饰paper的,可以不看,抓主干部分：Can paper be recycled?变为陈述句就是：Paper can be recycled.这样好理解了吧.
2.本句含义：他在考虑下面该说些甚么.
think about “考虑”,固定搭配；think of更侧重于“回想起来”,不符合题意.
3.本句含义：请保守这个秘密,只有你知,我知.
between...and...两者之间；若三者或三者以上之间用among