At a chemical synapse, what causes theion channels on the receiving cell's membrane toclose?
A) The channels get plugged up withexcessive neurotransmitters.
B) Sodium ions rush back out of thereceiving cell, causing the channel to close.
C) No additional neurotransmitters areproduced.
D) The neurotransmitter is broken down byan enzyme.
E) The synaptic cleft pinches together, squeezing the channels closed.