As you transfer heat into the frictionless piston-cylinder system, the mass M rises. The thermal energy transferred into the system equals the change in the

A. internal energy of the system.
B. internal energy of the system plus the change in the mass's gravitational potential energy.
C. internal energy of the system minus the change in the mass's gravitational potential energy.
D. mass's gravitational potential energy minus the change in the internal energy of the system.