The total area of the walls, windows, and ceiling of an apartment is 200 m2. The average thermal conductivity of the walls, windows, and ceiling is 0.13W/(m·°C), and their average thickness is 0.10 m. Assuming that all the heat loss is by conduction through the walls, windows, and ceiling. How much energy is necessary to keep the apartment at a steady 20 °C for 24 hours if the outside temperature is 0 degrees celsius?

a. 1.2 x 10^9 J.
b. 2.9 x 10^9 J.
c. 4.5 x 10^8 J.
d. 9.3 x 10^9 J.
e. 7.6 x 10^8 J.