题目

The door to the secret garden was locked ____________ years ago. A . nine B . eight C . ten D . eleven 答案：C18．已知$\overrightarrow{a}$，$\overrightarrow{b}$是单位向量，且$\overrightarrow{a}•\overrightarrow{b}$=-$\frac{1}{2}$．若平面向量$\overrightarrowhttp://pic.7249.cn$满足$\overrightarrowhttp://pic.7249.cn•\overrightarrow{a}$=$\overrightarrowhttp://pic.7249.cn•\overrightarrow{b}$=$\frac{1}{2}$，则|$\overrightarrowhttp://pic.7249.cn$|=（　　）A．2B．$\sqrt{2}$C．1D．$\frac{1}{2}$