The diagram shows a chemical synapse.
Which of the following statements, A to D, correctly describes the structures or events occurring at this synapse?
The arrival of an action potential causes structure P to close.
Structure Q fuses with the presynaptic membrane to release neurotransmitters by exocytosis.
Structure R is a voltage-gated sodium channel on the postsynaptic membrane.
Neurotransmitter molecules are actively transported across the synaptic cleft to reach structure R.