(5 points) A nxn matrix A is said to be nonsingular, if
 (1) A is non-invertible.
 (2) there exists a multiplicative inverse matix B such that A is row cquivalent to B.
 (3) the determinate of A is nonzero.
 (4) Ax=O where x is nonzero vector.
 (5) A is row equivalent to L