Cindy and Her Husband Are Trying to Have a Baby

Cindy and her husband are trying to have a baby. Based on the calendar method of birth control, when is the best time (approximately) for them to have intercourse in order to conceive?

A) one week after the beginning of menstruation
B) two weeks after the beginning of menstruation
C) three weeks after the beginning of menstruation
D) four weeks after the beginning of menstruation