Neuron a and Neuron B Synapse with Neuron C

Neuron A and Neuron B synapse with Neuron C. You are measuring the electrical activity in Neuron C. When Neuron A fires, an action potential is observed in Neuron C.
But when Neuron A and Neuron B fire at the same time, there is no change in the electrical activity of Neuron C. How can we explain this?

A) Neuron B depolarizes the postsynaptic membrane.
B) This would be a measurement error.
C) Neuron B generates a graded potential that cancels out the graded potential of Neuron A.
D) Neuron B uses serotonin as the neurotransmitter.
E) Neuron A hyperpolarizes the postsynaptic membrane.