13. Find all local extreme values of the given function and identify each as a local maximum, loca
f(x,y) = x^2 + 9xy + y^2
A. f(9,9) = 891, local maximum
B. f(0,0) = 0, saddle point
C. f(0,0) = 0, saddle point; f(9,9) = 891, local maximum
D. f(9,0) = 81, local minimum; f(0,9) = 81, local minimum