Final answer:
Matrix B must have 2 rows to result in a 2x9 matrix when multiplied by matrix C. The number of columns in matrix B and rows in matrix C remains undetermined without additional information.
Step-by-step explanation:
To determine how many rows matrix B has, we need to understand the rules of matrix multiplication. If matrix BC is a 2x9 matrix, this tells us that the result of the multiplication of matrix B with matrix C gives us a matrix with 2 rows and 9 columns.
The number of rows in the resulting matrix (matrix BC) comes from the number of rows in the first matrix (matrix B). Hence, matrix B must have 2 rows. However, to give a complete answer, we need to know the number of columns in matrix B and rows in matrix C because for matrix multiplication to be possible, the number of columns in the first matrix must equal the number of rows in the second matrix (matrix C).
Without information about matrix C, we can only conclude that matrix B must have 2 rows to be consistent with the 2x9 dimensions of matrix BC. The exact number of columns in matrix B (and accordingly, the number of rows in matrix C) cannot be determined from the information given.