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

the morning of my mother’s birthday, I usually help my mother do some housework.
A. In
B. At
C. During
D. On

A. InB. AtC. 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。

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