题目

 I’ll go to the library as soon as I finish what I _________.    A. was doing         B. am doing         C. have done          D. had been doing 答案：B11．在直角坐标系中，以原点为极点，x轴的正半轴为极轴建立坐标系．已知曲线C：ρsin2θ=2acosθ（a＞0），过点P（-2，-4）且倾斜角为$\frac{π}{4}$的直线l与曲线C分别交于M，N两点．（1）写出曲线C的直角坐标方程和直线l的参数方程；（2）若|PM|，|MN|，|PN|成等比数列，求a的值．