Two spheres of identical diameters (D=0.2 m) of different materials (magnesium: density =1740 kg/m, Cp=1024, k=156, and porcelain: density =2300 kg/m, Cp=750, k=2.2) are initially at room temperature (30 degrees C). Suddenly, they are placed in an oven at 600 degrees C, in which the exchange coefficient h=100. a) Calculate the Biot number for both cases. b) From the Biot numbers, indicate which calculation method can be used to calculate the temperature evolution of the spheres. c) Calculate the temperature at the center of the magnesium sphere after 10 minutes. d) After the ceramic sphere has been heated long enough to reach a uniform temperature of 600C, remove it from the furnace and immerse it in an oil bath at 100C, with an exchange coefficient of 3000 K. Calculate its core temperature after 5 minutes' cooling.