Which of the following is not correct? (A) If A, B are two matrices such that AB = A and BA=B, then A and B are idempotent. (B) Every skew - symmetric matrix of odd order is invertible. (C) If A is a non-singular matrix, then A-1 is skew-symmetric. (D) The adjoint of a diagonal matrix is a diagonal matrix. Choose the correct answer from the options given below :