15. Two metal plates are connected to a battery and a current meter. When a light beam with tunable energy E is shined on one of the plates, the values read by the current meter is plotted as a function of E. E0 corresponds to the photon density of 1012cm-3and the photon wavelength of 200 nm. When the incident photon wavelength is increased to 400 nm, what is the photon density to obtain the current value of 5 μA?(A)2✖1012 cm-3(B)4✖1012 cm-3(C)8✖1022cm-3(D)1012-3(E)None of the above