Which statement about activation of heart muscle cells is TRUE?   A when a person is resting only a few small motor units in each ventricle are activated in each heartbeat   B all of the muscle cells in the ventricles depolarize and contract together because they are connected by gap junctions   C motor neurons must synapse with all of the muscle cells in the ventricles at the same time or fibrillation will occur   D all of the heart muscle cells have unstable resting membrane potenitals and slowly depolarize to threshold at the same rate

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