Activation of M2Rs by ACh leads to a decrease in heart rate. Which of the following best explains this effect? a) Decreases the level of intracellular cAMP, thereby decreasing the magnitude of the funny current which is carried by a cyclic nucleotide-gated channel. b) Hyperpolarization prolongs the amount of time it takes phase 4 depolarization to reach threshold. c) Opening of potassium channels in the membrane leads to a decrease in membrane resistance and hence a decrease in heart rate.