题目

 He is a talkative man, but he didn't say a word in today's discussion. I am not sure how to______ his silence.A. read                     B. master                    C. grasp                       D. treat 答案：A选修4-4：坐标系与参数方程已知曲线C的参数方程为x=sinαy=cos2α，α∈[0，2π)，曲线D的极坐标方程为ρsin(θ+π4)=2a．（1）将曲线C的参数方程化为普通方程；（2）试确定实数a的取值范围，使曲线C与曲线D有公共点．