Neural messages are carried across the synaptic gap by chemical substances called neurons
neurotransmitters
nerves
synapses

They’re called neurotransmitters.

Ithink its a im not sure 

Horizontal asymptote at  tells us that the original graph has been translated down.since  ,  are positive.  also we can say that the point  has a smaller  -coordinate than  .this means that the  transformed exponential function is increasing as it goes to the right. as it goes from  to , the value of the exponential function increases. it goes from to something bigger, . therefore, as  . the left hand would be the side that approaches the horizontal asymptote.so the answer is d