The surface area of a giraffe is given by $f(M) = k M^{2/3}$, where $M$ is the body mass, and the constant of proportionality $k$ is a positive number that is determined by the general body shape of giraffes. Which of the following is a true statement? Be sure you can explain your answer in algebraic terms.

A. The surface area for a giraffe of body mass 60 kilograms is smaller than for a giraffe of 70 kilograms.
B. The surface area for a giraffe of body mass 60 kilograms cannot be compared to a giraffe of 70 kilograms because $k$ is not known.
C. The surface area for a giraffe of body mass 60 kilograms is larger than for a giraffe of 70 kilograms.
D. The surface area for a giraffe of body mass 60 kilograms is the same as for a giraffe of 70 kilograms.
E. None of the above