The arrival of graded potentials in a postsynaptic neuron may result in which of the following?

a) Opening of voltage-gated Na+ channels in the post-synaptic neuron.
b) The lack of an action potential in the post-synaptic neuron.
c) Summation of excitatory post-synaptic potentials that bring the cell to threshold.
d) Spatial summation of excitatory post-synaptic potentials that do not trigger an action potential.
e) All of the above
f) None of the above