At the Frog NMJ, Why Does Lowering the Extracellular Calcium

At the frog NMJ, why does lowering the extracellular calcium and adding extracellular magnesium not only reduces the amplitude of EPPs on average, but also leads to the occasional failure of the nerve terminal to release transmitter after nerve stimulation?

A) Occasionally the nerve terminal fails to trigger an action potential after nerve stimulation.
B) The reduced calcium and increased magnesium reduces the probability of transmitter release so much that occasionally there is a statistical chance of failure to release any transmitter.
C) The reduced calcium and increased magnesium reduces the probability of transmitter release to zero.
D) The reduced calcium and increased magnesium can also block the postsynaptic acetylcholine receptor.
E) The reduced calcium and increased magnesium increases the activity of the acetylcholinesterase in the synaptic cleft.