The function below models the correlation between the number of hours a plant is kept in sunlight (x) and the height (y), in mm, to which it grows:
y = 4 + 2x
What does the slope of this function represent?

A)The original height of the plant was 4 mm.
B)The original height of the plant was 2 mm.
C)The height of the plant increases by 2 mm for every hour of sunlight it receives.
D)The height of the plant increases by 4 mm for every hour of sunlight it receives.