A.         B.3          C.2          D.

A.{x|x≤2ab或x≥a²＋b²}                                      B.{x|x＜2ab或x≥a²＋b²}

C.{x|x＜2ab或x＞a²＋b²}                                      D.{x|2ab＜x≤a²＋b²}