The rhythm method of temporary contraception is based on which feature of the menstrual cycle?

• A
  Consecutive menstrual cycles are separated by $28$ days.
• B
  The luteal phase of the menstrual cycle has a duration of $14$ days.
• C
  $A$ week after ovulation is the only safe period for sexual intercourse.
• D
  $A$ week before and after ovulation are safe periods for sexual intercourse as the chances of fertilization are less.