Critical Reasoning

A birth is more likely to be difficult when the mother is over the age of 40 than when she is younger. Regardless
of the mother's age, a person whose birth was difficult is more likely to be ambidextrous than is a person whose
birth was not difficult. Since other causes of ambidexterity are not related to the mother's age, there must be
more ambidextrous people who were born to women over 40 than there are ambidextrous people who were born
to younger women. The argument is most vulnerable to which one of the following criticisms?
A. It assumes what it sets out to establish.
B. It overlooks the possibility that fewer children are born to women over 40 than to women under 40.
C. It fails to specify what percentage of people in the population as a whole are ambidextrous.
D. It does not state how old a child must be before its handedness can be determined.
E. It neglects to explain how difficulties during birth can result in a child's ambidexterity.

OA is B

but whats wrong with A ?