Removal of External Ca²⁺ to Levels Below About 0

Multiple Choice

Removal of external Ca²⁺ to levels below about 0.2 mM result in no discernible force from the heart. This is because

A) There is no cardiac action potential without extracellular Ca²⁺
B) There is no calcium-induced-calcium release without extracellular Ca²⁺
C) Ca²⁺ cannot enter over the NCX to activate the myofilaments
D) Ca²⁺ is pumped out of the cell by the PMCA
E) TnC is insensitive to Ca²⁺ when the heart is relaxed