Which of the Following Statements About Oral Contraceptives Is TRUE

Which of the following statements about oral contraceptives is TRUE?

A) They are effective only because they prevent ovulation.
B) Full protection is provided by oral contraceptives after three days of use.
C) The most common type of oral contraceptive contains both estrogen and progesterone.
D) A significant risk of pregnancy exists even if oral contraceptives are taken as directeD.