Barth Borrows Funds from City Bank to Pay His Tuition

Barth borrows funds from City Bank to pay his tuition. His mother Debi co-signs the credit application. After the loan agreement is signed, Barth agrees to a higher rate of interest without telling Debi. She is

A) discharged from the agreement.
B) liable at the higher rate of interest.
C) liable at the lower rate of interest.
D) liable for the principal only.