The force-frequency relation in the heart (the relation between frequency of stimulation and strength of the heart) is caused principally by

A) Stretch of the cardiac muscle with Increasing frequency of stimulation
B) Increased in the sensit ivity of the myofilaments to Ca₂+
C) Phosphorylation of TnC
D) Increased [Ca₂+] in the mitochondria
E) Increased [Ca₂+] transients due to loading of the SR