Seeing the happy _____ of children playing in the park, I’m full of joy and confidence in the future of our country.  

A．sight

B．scene

C．view

D．sign

动点P到两定点A（a，0），B（-a，0）连线的斜率的乘积为k，试求点P的轨迹方程，并讨论轨迹是什么曲线？