6. (6 points) Below is a contour plot of the C2 function f : R2 - R and the origin a = (0, 0) E R2. Determine whether each quantity is positive, negative, or zero. Select the most plausible answer. No justification is necessary. Fill in EXACTLY ONE circle. (unfilled O filled .) 2 3 4 3 (6a) f(a) O positive O negative O zero (6b) ay (a) O positive O negative O zero (6c) ax (a) O positive O negative O zero a (6d) a2f (a) O positive O negative O zero a2 f (6e) ayax -(a) O positive O negative O zero a2 f (6f) (a) O positive O negative O zero axay