Since A is 5 x 3 matrix, the matrixis 3 x 5 matrix (3 rows and 5 columns). Since the matrix product is defined, it implies that matrix B has 5 rows. Since the matrix C is 4 x 5 matrix and the matrix product BC is defined, it implies that matrix B has 4 columns. ANSWER. Matrix B has 5 rows and 4 columns, i.e. is 5 x 4 matrix.