47: Teens should find counsellors when they have problems.

("should" thay thế cho "necessary")

48: Our school hasn't introduced a special program for studens with disabilities since 2009.

("since" thay thế cho "in")

49: They haven't taken part in a charity campaign for 2 years.

("for" thay thế cho "was")

50: I would advise all parents should listen to their children's problem with sympathetic attitude.

("should" thêm vào dành cho lời khuyên)

Đáp án:   47.They should find counsellors when they have problems.

                48.Our school has introduced a special progam for students with disabilities since 2009.

                49.They have taken part in a charity campaign for 2 years.

                50.I should advise all parents to listen to their children's problems with sympathetics attitude.