Which answer is FASLE. cAMP is the second messenger for a number
of regulatory systems. Factors that explain how so many types of
signals can be mediated by cAMP include:

A. signals that open a pore in the cell membrane, which
transports cAMP extracellularly

B. signals that stimulate adenylyl cyclase to increase the
concentration of cAMP in the cell

C. adaptor proteins that hold tether other proteins that
function in concert, thus confining the cAMP signal to a specific
region of the cell