Don't throw away some old things．We can____them．We can put our pens and pencils in____．They will be nice____．We can put flowers in some bottles；they will be nice____．We can stick some____together and put our cards in them．How interesting!

下列生态系统中，具有提供并净化水源、蓄洪抗旱作用的是（    ）

We're going swimming __________ten o'clock.  

在复平面内，复数$\omega =-\frac{1}{2}+\frac{\sqrt{3}}{2}i$对应的向量为$\overrightarrow{OA}$，复数${\omega }^2$对应的向量为$\overrightarrow{OB}$．那么向量$\overrightarrow{AB}$对应的复数是（　　）