Acetylcholine binds to a GPCR on heart muscle, making the heart beat more slowly.The activated receptor stimulates a G protein, which opens a K⁺ channel in the plasma membrane, as shown in Figure 16-13.Which of the following would enhance this effect of the acetylcholine? Figure 16-13

A)addition of a high concentration of a nonhydrolyzable analog of GTP
B)addition of a drug that prevents the α subunit from exchanging GDP for GTP
C)mutations in the acetylcholine receptor that weaken the interaction between the receptor and acetylcholine
D)mutations in the acetylcholine receptor that weaken the interaction between the receptor and the G protein