For 3i^3 matrices M and N, which of the following statement(s) is (are) NOT correct ?
(A) NTMN is symmetric or skew symmetric, according as M is symmetric or skew symmetric
(B) MN, NM is skew symmetric for all symmetric matrices M and N
(C) MN is symmetric for all symmetric matrices M and N
(D) (adj M) (adj N) = adj(MN) for all invertible matrices M and N