The only possibility of heat flow in a thermos flask is through its cork which is 75 cm2 in area and 5 cm thick. Its thermal conductivity is 0.0075 cal/cmsecoC. The outside temperature is 40oC and latent heat of ice is 80 cal g?1. Time taken by 500 g of ice at 0oC in the flask to melt into water at 0oC is [CPMT 1974, 78; MNR 1983]

• A.            2.47 hr
• B.            4.27 hr
• C.            7.42 hr
• D.            4.72 hr