A least squares regression analysis of average gas mileage versus speed (mph) produces the following residual plot:
Based on the residual plot, which of the following statement(s) must be TRUE?
I. The relationship between speed and average gas mileage is non-linear.
II. The average gas mileage decreased as speed increased from $35$ mph to $60$ mph.
III. The regression equation would overestimate the average miles per gallon the car gets when the speed is $10$ miles per hour.