For the α subunit of a trimeric G protein, …

A) a G-protein-coupled receptor (GPCR) acts as a guanine nucleotide exchange factor (GEF), whereas a regulator of G protein signaling (RGS) can act as a GTPase-activating protein (GAP).
B) a GPCR acts as a GAP, whereas an RGS can act as a GEF.
C) both a GPCR and an RGS can act as a GEF.
D) both a GPCR and an RGS can act as a GAP.