When did people do house cleaning in the past（）

A.People used to do house cleaning on the 20th day of the 12th month.

B.People used to do house cleaning on the 21st day of the 12th month.

C.People used to do house cleaning on the 20th day of the 11th month.

D.People used to do house cleaning on the 21st day of the 11th month.

正确答案：A