choose the one answer that best completes the sentence.

choose the one answer that best completes the sentence.
16.Open your dictionary .You can find the word 'psychology'___under'P' ,after the word 'psychologist'.
A.listing B.listed C.lists D.to list
17.Jack is one of those men who I am sure always do ___best even in the most difficult situations.
A.his B.their C.your D.one's
18.A:Mom ,my holiday abroad cost a lot of money .
B:Did it?Well,it's all right ___you enjoyed yourself.
A.unless B.in case C.as far as D.as long as
19.___the Browns saw the house ,they liked it.
A.The moment B.As C.While D.Since
20.If human beings had been a bit less greedy and cruel,more birds and animals ___dying out.
A.ought to avoid B.could have been avoided C.should have avoided D.might have avoided