8. (5%) Supposeare row equivalent a，b，c are3 nonzero numbers. What is a + b? (A)0 (B) 2 (C) 3 (D) 4 (E) 5

4. (5%) Let A be an m by n matrix. Which statement is TRUE? (A) The null space of A is a subspace of the column space of AT (B) The dimension of the null space of A plus the dimension of the row space of A is (C) The null space of AT equals column space of A (D) The column space of A is orthogonal to the row space of AT (E) The direct sum of the row space of A and the null space of AT is Rm