I was born （ ）a sunny day.A.on B.at C.in D.for 应该选用哪一个介词?

英语里表示“在”的时间介词：
1)in表示较长时间,如世纪、朝代、时代、年、季节、月及一般（非特指）的早、中、晚等.
如 in the 20th century,in the 1950s,in 1989,in summer,in January,in the morning,in one’s life ,in one’s thirties等.
2)on表示具体某一天及其早、中、晚.
如,on a sunny day,on May 1st,on Monday,on New Year’s Day,on a cold night in January,on a fine morning,on Sunday afternoon等.
3)at表示某一时刻或较短暂的时间,或泛指圣诞节,复活节等.
如at 3:20,at this time of year,at the beginning of,at the end of …,at the age of …,at Christmas,at night,at noon,at this moment等.
题目中a sunny day为具体的某一天,所以选A.on