Let A be an m x n matrix, x is in Rn and b is in Rm. which of the following below is/are true?
A. a matrix equation Ax=b has a solution if and only if b is in the Span of the columns of A
B. a matrix equation Ax=b has a solution if and only if b is in the span of the columns of A
C. columns of A span the whole Rm if and only if Ax-b has a solution for any b in Rm
D. Ax=b has a solution for any b in Rm if and only if A has a pivot position in every row
E. Ax=b has a solution for every b in Rm if and only if rank(A)=n