If a Woman Has a Somewhat Irregular Menstrual Cycle,why Is

If a woman has a somewhat irregular menstrual cycle,why is she ill-advised to use the rhythm method?

A) It is impossible to calculate when ovulation occurs,and therefore impossible to determine which days are "safe."
B) She may end up having intercourse at the beginning of menstruation,thereby increasing the likelihood of conception.
C) Worrying about the possibility of pregnancy will prevent both her and her partner from being sexually satisfied.
D) An irregular menstrual cycle indicates problems with fertility,and these should be evaluated before any method of contraception is used.