When a G protein is activated in a signal transduction pathway,
A. GDP is released from the G protein, and GTP occupies the nucleotide-binding site.
B. GTP is released from the G protein, and GDP occupies the nucleotide-binding site.
C. cGMP occupies the otherwise empty nucleotide-binding site on the G protein.
D. cGMP leaves the otherwise occupied nucleotide-binding site on the G protein.
E. GDP causes a conformational change in the G protein.