(a) 3, 2, 3, 4 (b) 4, , (C) 0, 3, 4, (a) l, Exercise 3. For real symmetric matrices A and B, which of the following is true? (a) AB is symmetric matrix. (b) AB= BA (I1T-JAM 2005) (c) All eigenvalues of AB are real if AB = BA. (d) AB is invertible if either A is invertible or B is invertible.