Andrea is a MHIC$-$licensed contractor for home improvements in Maryland. Derrick calls Andrea about installing a new bathroom in his basement. They sign a written agreement that requires a deposit of $\frac{1}{3^{rd}}$ of the contract price at contract signing, $\frac{1}{3^{rd}}$ at substantial completion, and the balance due on acceptance. A specification is attached that details the scope of work and the location of the bathroom. After Andrea is nearly completed with the project, Derrick calls and tells her that he thinks the bathroom would look better on the second floor. Which, if any, of the following is true?

a$)$ Andrea can only recover in quasi contract against Derrick.

b$)$ Derrick is liable if he refuses to pay the balance owed on the agreement.

c$)$ Andrea is liable if she refuses to move the bathroom to the second floor.

d$)$ Derrick must be subjectively satisfied with Andrea's work as a condition of payment.