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.