57.　　 I am sorry for 　___________ (keep) you ___________ (wait) so long.

56.　　 Our government has done something ___________ (save) water.

55.　　 They don’t have enough bamboo ___________ (eat).

54.　　 We should do our best 　___________ (protect) them.

53.　　 We are supposed ___________ (do) so.

52.　　 I think ___________ (recycle) is useful.

51.　　 His job is　 ___________ (look) after the children.

50.　　 Easier　 ___________ (say) than ___________ (do).

49.　　 We must stop them from　 ___________ (pour) waste water into the rivers.

48.　　 You must stop ___________ (make) noise.