Answer:
D.
Explanation:
The constant rate of change has to represent a linear function, where the constant represents the slope of the linear function.
Basically, the constant rate of change tells about the change of the dependent variable regarding the independent variable. So, there has to be a linear relation which can be proportional, that is, the increase of one variable ensure the increase of the other variable.
So, in this case, option D shows a possible proportional relationship, because more airports visited implies more distance traveled, if one increase, the other one will too.