An iron rod of length 2m and cross section area of 50 X  , is stretched by 0.5 mm, when a mass of 250 kg is hung from its lower end. Young's modulus of the iron rod is-

(a) $19.6×{10}^{10}N/{m}^{2}$                         (b) $19.6×{10}^{15}N/{m}^{2}$

(c) $19.6×{10}^{18}N/{m}^{2}$                         (d) $19.6×{10}^{20}N/{m}^{2}$