Movements of the Body Are Usually Smooth and Occur at Differing

Movements of the body are usually smooth and occur at differing rates because

A) they are a mixture of isotonic and isometric contractions.
B) motor units contract out of phase at their own particular rates.
C) most muscle contractions closely resemble individual muscle twitches.
D) muscles of different sizes contract at different rates.
E) of the all-or-none principle.