How were extracellular ion concentrations manipulated in the experiments supporting the theory of quantal release of neurotransmitters?

A) Ca²⁺ was increased in the extracellular fluid so that action potentials would release many vesicles.
B) Ca²⁺ was absent in the extracellular fluid so that action potentials would release no vesicles.
C) Mg²⁺ replaced Ca²⁺ in the extracellular fluid so that action potentials would release few vesicles.
D) Mg²⁺ replaced Ca²⁺ in the extracellular fluid so that action potentials would release many vesicles.