A is an n×n matrix. Check the true statements below:

A. To find the eigenvalues of A, reduce A to echelon form.
B. A number c is an eigenvalue of A if and only if the equation (A−cI)x=0 has a nontrivial solution x.
C. A matrix A is not invertible if and only if 0 is an eigenvalue of A.
D. Finding an eigenvector of A might be difficult, but checking whether a given vector is in fact an eigenvector is easy.
E. If Ax=λx for some vector x, then λ is an eigenvalue of A.