An oscilloscope is used to record the membrane potential ($V_m$) at the axon hillock of a postsynaptic neurone under different stimulation conditions. Presynaptic neurones A and B both synapse onto the dendrites of this postsynaptic neurone. The experimental setup and the resulting electrical traces are shown below.

Which of the statements about synaptic transmission and the electrical traces is/are correct?

1. The small depolarisations shown in Section I are graded excitatory postsynaptic potentials (EPSPs) whose peak amplitude depends on the concentration of neurotransmitter released into the synaptic cleft.
2. The event in Section II illustrates temporal summation, where multiple action potentials arriving in rapid succession from a single presynaptic neurone can depolarise the postsynaptic membrane to the threshold potential.
3. The peak amplitude of the action potentials in Section II and Section III is directly proportional to the total number of presynaptic neurones activated.