题干
I called ________ his house this morning, but he wasn’t in. So I left a message telling him I would call______ him the next day.
A：on; o
B：at; at
C：at; o
D：on; at
答案(点此获取答案解析）
C
；它不可以发生的反应类型有：①取代②加成③消去④水解⑤酯化⑥中和⑦氧化⑧加聚
(n∈N*)，若前n项的和为
，则项数为