题目

.已知为正数，，其中是常数，且的最小值是，满足条件的点是椭圆一弦的中点，则此弦所在的直线方程为(     )     A．  B． C.．  D． 答案：D----It’s traditional in China to             firecrackers on Spring Festival. ----But the serious smog（雾霾）nowadays needs us to stop it.A．give outB．set offC．put offD．show off