39. ask for 要求

40. attach importance to 重视

41. pay attention to重视catch one's attention引起…注意/重视

42. available可得/买到的

There is no ticket available.

43.average平均on（the） ~

44. be aware of = realize意识到be sure一定要，肯定

He apologized to us for the mistake as soon as he was aware of it.

Be sure not to be late for the meeting.

I'm sure the play will be a great success.

45. back up支持

46. background背景

47. base ~…on…把…建立在…基础之上

His article is based on the research.

48. bear忍受，容忍同义词：

endure tolerate stand put up with

She cannot bear to see the child in pains.

49. do/try one's best

He said he would try his best to carry out the plan.

50. because连词：There will be no meeting because the manager will be on the vacation next week.

because of介词：His wedding will be put off because of his father's death.