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.

一节干电池的电压为______伏．家庭电路中，电冰箱与电视机是______的（选填“串联”或“并联”），家中电灯工作时将______能转化为光能．