A nurse instructs a prenatal class about the importance of doing Kegel exercises frequently. Kegel exercises help to （）.
A．promote better breathing by strengthening the diaphragm muscle.
B．maintain good perineal muscle tone by tightening the pubococcygeus muscle.
C．minimize leg cramps by strengthening the calf muscles.
D．prepare the mother for pushing by strengthening the abdominal muscles.
