[A] : Upon stimulation of a muscle cell, cal- cium ions `(Ca^(++))` are released from the sar- coplasmic reticulum.
[R]: In reacting with a protein complex, `Ca^(++)` unmasks myosin binding sites on the ac-tin filaments.
A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false