For 3 × 3 matrices M and N, which of the following statement(s) is (are) NOT correct?
(A) NᵀMN is symmetric or skew-symmetric, according to 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