Jack and Anna are good friends. They are Chinese. Jack likes sports very much. He can play volleyball , basketball and football . Anna likes music very much . She can't play ball games. But they both like playing on computers . They like eggs , bread and milk for breakfast . For lunch , Anna likes eating vegetables and fruit , but Jack likes chicken and fish . They both like hamburgers .But they don't like ice cream or rice.

a．如图甲所示，将铁笼系于槽钢上的B点，当吊车吊钩在槽钢上的悬吊点移至O点时，槽钢在水平位置平衡．

b．将大象引入铁笼，保持吊钩悬吊点O点和铁笼悬挂点B点的位置不变．用弹簧测力计在A点竖直向下拉住槽钢的另一端，使之再次在水平位置平衡，如图乙所示．测得OB=8cm，OA=9m，弹簧测力计的示数为200N．根据上述数据测出了大象的重力．

把$\sqrt{\frac{8}{9}}$化为最简二次根式是（　　）