题目

－Where is my sister, mum? I haven’t seen her this morning.     －She _______ to the library.     A. has been     B. is going     C. has gone    D. will go 答案：C15．以t为参数的直线方程$\left\{\begin{array}{l}{x=-1+\frac{t}{2}}\\{y=2+\frac{\sqrt{3}}{2}t}\end{array}\right.$，M0（-1，2），M（x，y）是曲线上的定点和动点，则t的几何意义是（　　）A．M0MB．MM0C．|M0M|D．2$\sqrt{2}$