A postsynaptic neuron has three presynaptic inputs-from neurons X, Y, and Z. Stimulation of neuron X causes the postsynaptic neuron to depolarize by 0.5 mV. When X and Y are stimulated simultaneously, the postsynaptic neuron depolarizes by 1 mV. When X and Z are stimulated simultaneously, however, there is no change in the membrane potential of the postsynaptic neuron. What is most likely true about presynaptic neurons Y and Z?
A. Z is excitatory and Y is inhibitory.
B. They are both inhibitory.
C. They are both excitatory.
D. Y is excitatory and Z is inhibitory.
Answer: D
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