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, there is a depolarization observed in Neuron C, but not enough to generate an action potential. But when Neuron A and Neuron B fire at the same time, there is an action potential generated in Neuron C. How can we explain this?

A) Measurement error on the first observation.
B) The synaptic connection for Neuron B is actually at the axon.
C) Neuron A depolarizes the postsynaptic membrane, but Neuron B hyperpolarizes it.
D) Neuron A hyperpolarizes the postsynaptic membrane, but Neuron B depolarizes it.
E) The graded potentials from Neuron A and Neuron B sum together to reach the depolarization necessary for an action potential.