题目

设集合A＝{x|x2－x＋a＝0}，若∅A，求实数a的取值范围. 答案：解 ∵∅{x|x2－x＋a＝0}.∴{x|x2－x＋a＝0}≠∅. 即x2－x＋a＝0有实根.∴Δ＝(－1)2－4a≥0，得a≤. 所以实数a的取值范围是.The doctors and nurses did ________ they could to help the wounded. ［　　］ A．all thatB．all what C．all whichD．what that