What were you doing last Sunday morning? A. atB. inC. OverD. /

问题补充：

What were you doing last Sunday morning?

A. at

B. in

C. Over

D. /

答案：

A. at

B. in

C. Over

(答案→)D. /

解析：句意:上个星期天的早上你在做什么？at 通常用来表示具体时刻；in 表示泛指在早上、下午或晚上；over 表示在…期间。last Sunday morning ，上个星期天，前面不用介词。故选D。