68. Let x be a real number. Given 2 22 f ( ) 2( 2 1) 4 8 3 x xx xx = +−− −− , −≤ ≤ 2 1 x . Let the maximum of ( ) f x is M and the minimum is m. Evaluate M + m = (A) 0 (B) 3 (C) −3 (D) 6 11