The radius of the base of a cone is increasing at the rate of 3 cm/min and the altitude is decreasing at the rate of 4 cm/min. The rate of change of lateral surface when the radius is 7 cm and altitude is 24 cm is
a. 54 phi cm^2/min
b. 7 phi cm^2/min
c. 27 phi cm^2/min
d. none of these