15. Let A be an n x n matrix. Which of the following statements are true?
(A) A symmetric matix A is positive definite if and only if all the eigenvalues of A are positive.
(B) A is nonsingular if and only if detA=0.
(C) A is singular if and only if 0 is an eigenvalue of A.
(D) detA = λ1+ λ2 +...+ λn , where λi's are eigenvalues of A.
(E) None of the above.