Please Use the Following Information to Answer the Question(s) Below

Please use the following information to answer the question(s) below.
A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively. A third gene for the MN blood group has codominant alleles M and N.
-Which of these is a way that the sexual life cycle increases genetic variation in a species?

A) by allowing crossing over
B) by allowing an increase in cell number
C) by increasing gene stability
D) by conserving chromosomal gene order
E) by decreasing mutation frequency