The given diagram shows the axon terminal and synapse. Identify the correct pair of any two labels from $A$ to $D$.

• A
  $C$ - Neurotransmitter,$D$ - $Ca^{++}$
• B
  $A$ - Receptor,$C$ - Synaptic vesicle
• C
  $B$ - Synaptic cleft,$D$ - $K^+$
• D
  $A$ - Neurotransmitter,$B$ - Synaptic cleft