Which is correct and most complete?

a. In two stage RSM, interactions are never in the fitted model form.

b. In two stage RSM, finding lack of fit indicates more runs should be performed.

c. Two stage RSM cannot terminate with a full quadratic fitted model in all factors.

d. In two stage RSM, lack of fit is determined using a t-test.

e. All of the above are correct.

f. All of the above are correct except

(c) and (e).