Arrange the following events in proper sequence during impulse transmission across a chemical synapse:
$(a)$ Fusion of synaptic vesicles with the cell membrane of the presynaptic neuron.
$(b)$ $Ca^{++}$ channels open and $Ca^{++}$ diffuses into the synaptic knob from the extracellular fluid.
$(c)$ Neurotransmitter binds with receptors of the postsynaptic cell.
$(d)$ Release of neurotransmitter by exocytosis.

• A
  $d \rightarrow c \rightarrow a \rightarrow b$
• B
  $a \rightarrow c \rightarrow b \rightarrow d$
• C
  $b \rightarrow a \rightarrow d \rightarrow c$
• D
  $c \rightarrow a \rightarrow d \rightarrow b$