A cell's ATP-to-ADP ratio is most nearly equal to the ratio of its fluorescence emission intensities when Y32 is excited at:
A. 500 nm versus when it is excited at 420 nm.
B. 420 nm versus when it is excited at 500 nm.
C. 500 nm versus when it is in its ground state at 500 nm.
D. 420 nm versus when it is in its ground state at 420 nm