the morning of my mother’s birthday I usually help my mother do some housework.A. InB. A

问题补充：

the morning of my mother’s birthday, I usually help my mother do some housework.

A. In

B. At

C. During

D. On

答案：

A. In

B. At

C. During

(答案→)D. On

解析：考查介词，在具体某一天的早上，下午，晚上都是介词on，句意是;在妈妈生日的早上，我通常帮妈妈做家务。选D。