题目

已知点坐标为，，点在轴上，且，则点坐标为（ ）Ａ．         Ｂ．          Ｃ．         Ｄ． 答案：AEvery possible means _______to prevent the air pollution, but the sky is still not clear. A．are being usedB．have been usedC．has been usedD．had been used