51 standing

52 to have been built

53 doing

54 being helped

55 lying

56 buying

57 to convince

II.1b to seeing

2c risk climbing

3c buying him a book

4b to send

5c doing

6c opening

7c smoking

8c having

9d smoking

10c discovering

11b to smoke

12a to enter

13a stay

14a b&c are correct

15a laughing

16c to buy

17c to stay

18a doing

19d all are correct

20d I won't be

21b to finish/writing

51 standing

52 to have been built

53 doing

54 being helped

55 lying

56 buying

57 to convince

2 1 b 

2 c 

3 c 

4 b 

5 c

6 c 

7 c 

8 c

9 d 

10 c 

11 b 

12 a 

13 a

14 a 

15 a

16 c 

17 c

18 a

19 d

20 d 

21 b