The Dentist Is Pleased If, and Only If, I Both

Multiple Choice

The dentist is pleased if, and only if, I both brush and floss. The dentist is not pleased, but I brush. So, I don't floss.
-Working backward from the conclusion of this argument, which of the following is the most likely justification of the last step of the derivation?

A) MT
B) BMP
C) BMT
D) Simp
E) DS