Identify the problem(s) in each of the residual plotsshown

Step 1 of 4

a)  By observing residual plot for the straight-line model reveals a nonrandom

     pattern. The residuals exhibit a curved shape, with the residuals for the small

     values of x above the horizontal 0 (mean of the residual) line, the residuals

     corresponding to the middle values of x below the 0 line, and the residuals for the

     largest values of x again above the 0 line. The indication is that the mean value of

     the random error  within each of these ranges x (small, medium, large) may not

     be equal to 0. Such a pattern usually indicates that curvature needs to be added to

     the model.