To initiate a smooth muscle cell contraction, calcium enters the cell and binds to
A. troponin, which then activates myosin light-chain kinase.
B. calmodulin, which then binds to and activates myosin light-chain kinase.
C. myosin light-chain kinase, which then hydrolyzes ATP.
D. a phosphatase, which then removes the tropomyosin blockade.
E. a myosin head, which then activates calmodulin.
Answer: B
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