Some experts think the wolf's team spirit for survival is ____ is needed in today’s competitive culture. A. that B. what C. which D. how 答案:B(08年金华一中理) (15分) 动圆过定点且与直线相切,圆心的轨迹为曲线,过作曲线两条互相垂直的弦,设的中点分别为、。(1)求曲线的方程;(2)求证:直线必过定点;(3)分别以、为直径作圆,求两圆相交弦中点的轨迹方程。