A sphere made of iron has a diameter of 2.5 cm. Density of iron can be taken as 7897 kg/m3, specific heat of 0.452 kJ/kg.K and thermal conductivity as 73 W/m.K. The initial temperature of the sphere is 293 K. a) This sphere is suddenly thrown into a hot fluid and its surface temperature rises to 400 K. What is the time required for the center of the sphere to reach 350 K? b) When the surface of another sphere of the same size and temperature, but made of a different material, is raised to 400 K again, the time required for the temperature at its center to reach 350 K is measured as three times that required for the iron sphere, calculate the thermal diffusivity of this material.