The skin response in rats to different concentrations of a newly developed vaccine was measured in an experiment, resulting in the data, models, and computer output that follow.

a. Plot the straight-line, quadratic, and cubic equations on the scatter diagram for this data set.
b. Test sequentially for significant straight-line fit, for significant addition of X2, and for significant addition of X3 to the model.
c. Which of the three models do you recommend, and why?
Linear regression of Y on X

Quadratic regression of Y on X

Cubic regression of Y on X