I thick it's c but please don't put it if it is wrong
I think that it is geometry.
Answer:
Remember that the maximum rate of change of f at a point u is the length of the of the gradient vector evaluate in u, and the direction in which it occurs is in direction of the gradient vector evaluate in u.
The gradient vector of f is

Then, the maximum rate of change is
in the direction of (5,8).
7 years
First of all, half of 18 is 9, and if you divide 18 by 2.5, it results in 7.2. I hope this helps!