If A and B are two matrices such that AB is of order n × n, then which one of the following is correct?

How many matrices of different orders are possible with elements comprising all prime numbers less than 30?

Let $A = \left| {\begin{array}{*{20}{c}} p&q\\ r&s \end{array}} \right|$

where p, q, r and s are any four different prime numbers less than 20. What is the maximum value of the determinant?

If A and B are square matrices of order 2 such that det(AB) = det(BA), then which one of the following is correct?