Figure Q3-6 is from an experiment that helped show that neurotransmitter release is triggered by presynaptic Ca2+. In panel A, the presynaptic membrane potential was clamped at -25mV. Figure Q3-6 A. How did they know Ca2+ entered the presynaptic neuron? B. What happened in the postsynaptic neuron? C. In panel B, the presynaptic membrane potential was clamped at -50mV. Why was there no net current during the presynaptic depolarization? D. Why was there a current after the presynaptic potential was clamped back to -70mV? E. What happened to the delay in postsynaptic current with the +50mV presynaptic depolarization?