A function, fix), models the depth of water in a wading pool that is filling at a rate of 11 gallons per minute. Which of these describes why f(x) can be considered a linear function?
I. For every 1 minute interval, the depth of water in the wading pool increases by the same amount.
II. The function grows by equal factors every minute.
III. The slope of the function is constant.

a. I only
b. I and III only
c. II and III only
d. I, II, and III