12. The volume of a sphere is given by V = = mr3, with radius r. Which of the following represents the rate of change of the radius ? 1 dv 1 [A] = dv at [B] at 4TCr2 [C] 4 -173 . dr at at , dt [D] = 4mr2 . dr av = 412 at [E] dr 13. If f (x) = x5 + x+ 1, find (f-1)'(3). Note: The point (1,3) is on the graph of f. [A] 3 [B] 1 [C] 5x4 + 1 [D] [E]