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Ultraviolet radiations of 6.2 eV falls on an aluminium surface (work function 4.2 eV ). The kinetic energy in joules of the fastest electron emitted is approximately

(a) $3.2×{10}^{-21}$              (b) $3.2×{10}^{-19}$
(c) $3.2×{10}^{-17}$              (d) $3.2×{10}^{-15}$