For matrices A and B

For matrices A and B, we say that B is a submatrix of A if B is obtained by removing some columns and rows from A. For example, 1 2 4 is the submatrix of -8- -2 - -9 - obtained be removing the first and fourth rows and the second and fourth columns. Prove that if A has rank r with r > 0, then A has an r x r invertible subma- trix