抛物线y^2=2px(p>0)上有A(x1,y1`),B(x2,y2),C(x3,y3)三点，F是它的焦点，若AF，BF，CF的长成等差数列，则（ ）A。x1,x2,x3成等差数列 B.y1,y2,y3成等差数列 C.x1,x3,x2成等差数列D.y1,y3,y2成等差数列
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选A。x1,x2,x3成等差数列 :抛物线上一点到焦点的距离，等于到准线的距离：|AF| = x1 +p/2因此，若AF，BF，CF的长成等差数列，则，2*BF = AF +CF即，2*(x2 +p/2)=(x1 +p/2)+(x3 +p/2)== 2*x2 = x1 +x3== x1,x2,x3成等差数列