问题补充：
Dear Peter,
Thanks very much on inviting me to your birthday 86.
party on Sunday. Id like very much come but 87.
I had an examination on Monday morning. It is 88.
a very important exam but I can afford to 89.
fail it. Ill spend all the whole weekend reading 90.
and prepare for it. So Im really sorry that91.
I won be able to come in this time. Hope you 92.
can understand. Ill take this chance to wish93.
you wonderful time on your birthday. Happy94.
birthday, Peter, and many happy return of the day! 95.
Yours,
Li Ming86.on→for
答案：
(答案→)86.on→for
解析：
86.on→for考查介词用法。Thanks后接介词for表示为什么而感谢，后接动名词作宾语。
87.much后加to考查动词不定式的用法。Id like to do sth.的意思是“愿意做什么”。
88.had→will have考查动词时态。根据上下文可以确定此句的谓语动词是一般将来时。
89.but→and/so考查连词的用法。根据句意可知，此句是并列关系，而不是转折关系。改为and，表示“并且”；改为so，表示“因此”。
90.划掉all考查代词的用法。all一般不和whole连用。这句话的意思是“我将花费一周的时间阅读和准备(考试)。”
91. prepare→preparing考查非谓语动词的用法。preparing和reading是并列关系，构成spend some time(in) doing sth.句型。
92.划掉in考查介词的用法。this time的意思是“这次”，前面不用介词。
93.√
94.you后加a考查冠词的基本用法。time前有形容词时常加不定冠词。
95. return→returns考查名词的数。return在该句中是名词，根据前面的many可以确定它是复数名词。
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