Which of the following best explains why the use of an intrauterine device (IUD) increases the risk of developing pelvic inflammatory disease?

A) The absorbent cotton string of the IUD acts as a wick for bacteria to enter the uterus.
B) The use of a copper IUD weakens the user's immune system.
C) Women with an IUD are more likely than other women to have multiple sexual partners.
D) An IUD may cause irritation of the endometrium, allowing microorganisms to invade the uterine wall.

D