Let A, B and C be square matrices, which of the following statements is true? A. Ax=0 has only the trivial solution if and only if A can be written as the product of elementary matrices. B. Then A is row equivalent to B if and only if A is column equivalent to B. C. If , then A is singular. D. If AB=AC,then we have B=C. E. If A is nonsingular, then N(A)=N(AB), i.e. Ax=0 and ABx=0 have the same solution set.