Sun gives light at rate of $\;1400 Wm^{-2}\;$ of area perpendicular to direction of light . Assume $\;\lambda\;$ (sun light )= $\;6000 A^{0}\;.$ Then no.of Photons/sec arriving at $\;1m^2\;$ area at that part of earth is

Question

$(a)\;4.2\times10^{20}\qquad(b)\;4.2\times10^{22}\qquad(c)\;4.2\times10^{21}\qquad(d)\;4.2\times10^{23}$