Question

Which product is undefined?
A. HK, where His a 3 x 3 matrix and K is a 2 x 3 matrix
B. AB, where A is a 1 x 2 matrix and B is a 2 x 1 matrix
C. CD, where Cis a 3 x 1 matrix and D is a 1 x 2 matrix
D. NP, where N is a 2 x 1 matrix and Pis a 1 x 4 matrix