A difference between the mechanisms of cAMP and Ca²⁺ in signal transduction is that cAMP __________ and Ca²⁺ __________.

• A. is always present at high levels in the cytosol; is present at low levels in the absence of a signal
• B. is stored in the endoplasmic reticulum; is never stored in the cell
• C. is tyrosine-kinase-receptor linked; is G-protein-receptor linked
• D. enters the cell via a transmembrane protein channel; enters the cell by diffusing across the plasma membrane
• E. is synthesized by an enzyme in response to a signal; is released from intracellular stores