A Solar Heated House Loses About 7 $\degree$ C by the Solar Collectors, and the House Is Maintained

A solar heated house loses about 7.4 *10⁷ cal through its outer surfaces on a typical 24-hour winter day. What mass of storage rock is needed to provide this amount of heat if it is brought up to initial temperature of 62 $\degree$ C by the solar collectors, and the house is maintained at 20 $\degree$ C? (Specific heat of rock is 0.21 cal/g · $\degree$ C.)

A) 160 kg
B) 1200 kg
C) 6100 kg
D) 8400 kg