Approximately 15% of Couples in the United States Have Difficulty

Approximately 15% of couples in the United States have difficulty establishing a pregnancy. The causes for infertility are

A) most often due to problems on the woman's side.
B) most often due to problems on the man's side.
C) as equally likely to be due to problems on the man's side as on the woman's side.
D) most often unable to be identified medically.
E) most often due to societal pressure to become parents.