Which is correct and most complete?

a. If X is n × k with n > k, X′ (the transpose) is n × k.

b. If RSM is applied, (X′X)–1X′ cannot be calculated for quadratic model forms because X′X is a singular matrix.

c. Often, EIMSE designs are not available with fewer runs than CCDs or BBDs.

d. Central composite designs include fractional factorial, star, and center points.

e. All of the above are correct.

f. All of the above are correct except

(a) and (e).