练习：
1.A numbers B music C secrets D films
2.A appeals B appoint C appears D applies
3.A because B unless C although D still
4.A about B in C for D behind
5.A criminals B people C men D policemen
6.A when B until C before D than
7.A device B paper C file D document
8.A However B So C But D Nevertheless
9.A invisible B non-existent C visible D apparent
10.A card B keys C screen D speaker
11.A of B up C on D into
12.A upon B back C without D with
13.A was given B belonged C was owned D became
14.A Generally B Surprisingly C Disappointedly D Shortly
15.A explanation B discovery C book D list