When Mitt Romney Chose Paul Ryan as His Running Mate

When Mitt Romney chose Paul Ryan as his running mate in the 2012 election, he made a decision consistent with the traditional rule that says that vice-presidential candidates

A) are chosen for electoral reasons, such as winning a battleground state and/or providing ideological balance on the ticket.
B) are chosen on the basis of their policy expertise, especially foreign policy expertise.
C) are chosen because of their skills as campaigners.
D) are chosen on the basis of their stance on abortion rights.