1.has been on
2.has gone,will be
3.have studied
4.haven't heard
5.has worn
6.have,been
7.has gone
8.have found
did find
9.have been
10.haven't come
11.has taught
12.has been
13.have,littered
14.has,written
15.Has,cut
16.Have,spit
17.Has,studied
18.have,studied
19.have known
20.has worked