EXERCISE 2:
1, B. to go out --> going out
2, B. learning --> learn
3, A. on to hearing --> on hearing
4, D. to repair --> repairing
5, D. doing --> to do
6, A. Gets --> having got
7, A. not to help --> not helping
8, B. to unlocking --> to unlock
9, A. Open --> Opening
10, B. to use --> using
11, C. preventing --> to prevent
12, D. pollute --> unpollute
13, C. staying --> stay
14, B. setting --> set
15, C. visit --> to visit
16, B. enriching --> to enrich
17, C. beings --> to be
18, D. unavoidable --> unavoidabled
19, A. give --> giving
20, B. finished --> finish
21, C. eaten --> eating