Let f. Then, the rate of change in f at x as we change y but hold z fixed is ____, and the rate of change in f at y as we change z but hold x fixed is ____.
a) ∂f/∂x, ∂f/∂y
b) ∂f/∂y, ∂f/∂z
c) ∂f/∂z, ∂f/∂x
d) ∂f/∂x, ∂f/∂z