Our school is a green school．What makes it green？

    First，we have a lot of trees．There are trees in front of and behind every building．Some trees are green only in spring and summer．And some are green all year round．The trees make our school look like a big forest.

    And our school is a green one because we "go green". How do we go green？We have two green days every month．On green days，no students or teachers go to school by car．And we have clean﹣up Day every month．That day，students go to streets to pick up rubbish．Also，we have tree﹣planting Day every spring．We plant young trees on hills．We have three green clubs The members of the clubs look after those trees well．

    Green is not only a colour．It is about life．Please go green with us and make the world a more beautiful place．

下列说法中正确的是(    )

（I）求椭圆C的标准方程；

（II）过点F₁的直线l与该椭圆C交于M、N两点，且| $\overrightarrow{F_{2}M}$ + $\overrightarrow{F_2}$ N|= $\frac{2\sqrt{26}}{3}$ ，求直线l的方程．

（Ⅲ）是否存在圆心在原点的圆，使得该圆的任一条切线与椭圆C有两个交点A、B，且OA⊥OB？若存在，写出该圆的方程，否则，说明理由．