Which is correct about the contraction sequence of the heart?
A. The contraction and relaxation of all four chambers happen at the same time.
B. The right atrium and ventricle contract at the same time, and as they are relaxing, the left atrium and left ventricle begin to contract.
C. Contraction begins in the order: right atrium, then right ventricle, then left atrium, then left ventricle.
D. The two atria contract at the same time, and as they are relaxing, the two ventricles contract together.
E. The atria and ventricles begin their contractions at the exact same time, but the atrial contraction only lasts half as long as the ventricle contraction.
D
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