[20 P] If pure liquid lead is cooled to 80C(T) below its melting point, homogenous nucleation occurs, a) calculate the critical radius (r) and the activation free energy (G). b) calculate the required undercooling (T) in order to have a critical radius half in size found in part (a). (the latent heat of fusion and surface free energy are 237J/cm3 and 33107 J/cm2, respectively, and melting temperature of lead is 327C ). (Use the temperature unit as K in calculations )