Keys:
1—10 BCCAB CCDBB
11—20 DACBA ADBAC
21—30 ADBAD BCABD
31—40 CABAB BDCCA
41—50 DBACA CDABC
51—55 DBBCD
56. trains → train 57. it → which 58. on → to 59. see → read 60. such → so
61. when → until 62. wait → wait a 63. 正确 64. were → was
65. very → much / 去掉very
