51. (A)I will leave the party early (B)because(C) I must(D) study for my exam. 52. I (A)stayed up (B)late last night because I (C)mustn't go to school (D)on Sunday.
53. We (A)ought to not play football (B)as(C) it's raining(D) outside.
54. You (A) mustn't(B) to drive a car (C) if you (D) don't have a driving licence.
55. If you (A) want some useful (B)advice, you (C)have better talk to your parents (D) about your problem.

51.I will leave the party earl because i must study for my exam.

52.I late last night because i mustn't go to school on Sunday

53.We ought not to play football as it's raining outside

54.You mustn't to drive a car if you don't have  a driving licence

55.If you want some useful advice ,you must have better talk to your parents about your probem.

Explanation:

idk heyy

explanation:

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).