When a G Protein Is Activated in a Signal Transduction

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.