G Proteins and G-Protein-Linked Receptors

G proteins and G-protein-linked receptors

A) are found only in animal cells, and only embedded in or located just beneath the cell's membrane.
B) are found only in bacterial cells, embedded in the cell's plasma membrane only.
C) are thought to have evolved very early, because of their similar structure and function in a wide variety of modern organisms.
D) probably evolved from an adaptation of the citric acid cycle.
E) are not widespread in nature and were unimportant in the evolution of eukaryotes.