61. on the average (=on average， on an average)平均
62. (be) aware of (=be conscious of ， having knowledge or consciousness)意识到，知道。
63. at the back of (=behind)在…后面
64. in the back of在…后部(里面); on the back of在…后部(外面); be on one's back(=be ill in bed)卧病不起。
65. at one's back(=supporting or favoring sb.)支持，维护; have sb. at one’s back有…支持，有…作后台
