Write a function that takes two matrices (say, A and B) as arguments and return the matrix multiplication of A and B. Recall that in matrix multiplication, let C = A B, then Cijk Aik Bkj. Notice that in matrix multiplication, the number of columns in A must be the same as the number of rows in B. So in your program, if the inputs do not satisfy the condition, then provide an error message. Notice that, you are not allowed to use the R internal matrix multiplication function for this question, although you can verify your solution with it.