Identify the sequential steps occurring in a chemical synapse.

• A
  Impulse in axon $\rightarrow$ Entry of $Ca^{+2}$ into synaptic knob $\rightarrow$ Release of neurotransmitters $\rightarrow$ Neurotransmitters bind to specific receptors $\rightarrow$ Ion channels open $\rightarrow$ Generation of new potential in post-synaptic neuron.
• B
  Impulse in axon $\rightarrow$ Entry of $Na^{+}$ into synaptic knob $\rightarrow$ Release of neurotransmitters $\rightarrow$ Neurotransmitters bind to specific receptors $\rightarrow$ Ion channels open $\rightarrow$ Generation of new potential in pre-synaptic neuron.
• C
  Impulse in axon $\rightarrow$ Entry of $Ca^{+2}$ into synaptic knob $\rightarrow$ Release of neurotransmitters $\rightarrow$ Neurotransmitters bind to specific receptors $\rightarrow$ Ion channels close $\rightarrow$ Generation of new potential in post-synaptic neuron.
• D
  Impulse in axon $\rightarrow$ Entry of $Na^{+}$ into synaptic knob $\rightarrow$ Release of neurotransmitters $\rightarrow$ Neurotransmitters bind to specific receptors $\rightarrow$ Ion channels close $\rightarrow$ Generation of new potential in pre-synaptic neuron.