Brandon drew 4 circles, each with a different diameter, on a piece of paper. For each circle, he laid a piece of string along the outline and then measured the length of the string. His results are shown in the table below.

Circle Measurements

Diameter Length
of String
5 cm 15.7 cm
10 cm 31.4 cm
15 cm 47.1 cm
20 cm 62.8 cm

Brandon wrote the equation, y = kx, to model the relationship between the circumference and the diameter of the circle. Which of the following statements correctly describe what k represents in this equation?
k represents the radius
k represents the unit rate
k represents the area
k represents
A.
II and IV only
B.
I, II, and IV only
C.
I and III only
D.
I and IV only