When a 4 kg mass a hung vertically on a light spring that obeys Hooke's law, the spring stretches by 2 cm. The work required to be done by an external agent in stretching this spring by 5 cms will be (g=9.8 metres/sec^2).a. 4900 joule b. 2450 joule c. 0.495 joule d. 0.245 joule
