When Generic Drugs Enter the Market After the Patent for a Brand

When generic drugs enter the market after the patent for a brand name drug expires,the price of the brand name drug often increases.This is usually due to

A) the demand curve for the brand name drug shifting in but becoming more inelastic.
B) the demand curve for the brand name drug not shifting, but the marginal cost of producing the good increases.
C) the demand curve for the brand name drug shifting in but becoming more elastic.
D) the demand curve for the brand name drug not shifting, but loyal customers are willing to pay a higher price for the brand name drug.