哪句才對?謝謝!!!!!

(C)才對.
Benny ate a cake in the morning.
at 通常有特定時間, 例如 at 11.45am / at 3.pm. in 無特定時間. 例如 in the morning / in the afternoon

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Sentence C is correct. We should say "in" the morning not on the morning or at the morning.

一定係C. Benny ate a cake in the morning?

[1] 時間介系詞：at，in，on

※ 指一天裡的特定時間或時段

at + 時間 ( at 6 o`clock )

at + 時間 ( at 11:45 )

at + 時段 ( at midnight )

at + 時段 ( at lunchtime )

at + 時段 ( at sunset )

at + 時段 ( at weekend )

※ 指特定的日期或節日

on + 日期 ( on Monday )

on + 日期 ( on 12 May 1981 )

on + 特定節日 ( on Thanksgiving Day )

on + 特定節日 ( on my birthday )

※ 指特定月份、年份、季節、時段

in + 月份 ( in October )

in + 年份 ( in 1986 )

in + 年份 ( in the 18th century )

in + 季節 ( in the winter --- the可省略 )

in + 時段 ( in the past )

in + 時段 ( in the future --- the可省略 )

緊係 in the morning 啦 ~

C. Benny ate a cake in the morning?

C. Benny ate a cake in the morning