Suppose you're studying a particular synapse in vitro (in a dish). You stimulate an action potential in the presynaptic neuron while recording membrane potential in the postsynaptic neuron. However, when you do this, you see no obvious change in the membrane potential of the postsynaptic neuron, even though you're sure there's a synapse between the two cells.

Which of the following types of synapse could this be? (choose all that apply)
a. An excitatory synapse
b. An inhibitory synapse
c. An electrical synapse
d. A neuromodulatory synapse