At the Frog NMJ, Why Does Lowering the Extracellular Calcium

At the frog NMJ, why does lowering the extracellular calcium and adding extracellular magnesium reduce the amplitude of EPPs?

A) This results in a reduced probability of quantal release of transmitter due to reduced calcium influx.
B) This results in an increased probability of quantal release of transmitter due to increased magnesium influx.
C) This results in a reduced probability of quantal release of transmitter due to a reduced depolarization of the nerve terminal membrane.
D) This results in a reduced probability of quantal release of transmitter due to a increased depolarization of the nerve terminal membrane.
E) This results in a change in the nerve terminal input resistance which changes the magnitude of the EPP without a change in the probability of quantal release of transmitter.