For relaxation to occur,
A. extracellular calcium levels fall, calcium is pumped into the myofibril, and tropomyosin slides away from actin.
B. calcium leaves the sarcoplasmic reticulum, and troponin binds calcium and blocks the binding sites on actin.
C. calcium enters the myofibril, it detaches from myosin, and the binding sites on myosin become inactive.
D. sarcoplasm calcium levels fall, calcium is removed from troponin, and tropomyosin blocks binding sites on actin.
Answer: D
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