81. Teaching a pronunciation class to a mixed group of learners can ＿＿ a teacher with many challenging problems.
A. provide     B. produce     C. present     D. offer
82. We want our children to know that hard work＿＿＿＿＿.
A. comes off B. gives off     C. pays off    D. sees off
83. We can’t wait. We have to ＿＿＿＿the direction and the distance before we take action.
A. make out  B. figure out     C. think out  D. turn out
84. It＿＿＿＿to look after these naughty grandchildren of mine for a whole day.
A. put me down  B. drives me out    C. wears me out    D. pulls me through
85. We’ d better try to ＿＿＿＿with the experiment, I think. Now let’ s ＿＿＿ with it.
A. go through； go on B. go on；  go over  C. go over ； go through  D. go on；  go through
86. The host stood at the door and  ＿＿＿＿every guest a welcome.
A. nodded     B. dropped off     C. shook     D. moved
87. No one knows when XO was first discovered, or how it ＿＿＿to be such a