The f (x,y)=x4 – x4 +4xy + 5. has O A local minimum at (1,1). local maximum at (-1,-1) and saddle point (0,0). B-only saddle point at (0,0) Conly local maximum at (0,0). O D. local minimum at (1,1), (-1,-1) and saddle point at (0,0).