已知向量 $\overrightarrow{a}$ ， $\overrightarrow{b}$ 满足| $\overrightarrow{a}$ |=1，| $\overrightarrow{b}$ |= $\sqrt{3}$ ，|2 $\overrightarrow{a}$ + $\overrightarrow{b}$ |= $\sqrt{7}$ ，则 $\overrightarrow{b}$ 与 $\overrightarrow{a}$ ﹣ $\overrightarrow{b}$ 的夹角为（   ）

任务型阅读

    Have you taken a plane? Have you thought about the ways to survive(生存) in a plane crash(坠机)? Here are some tips for you. ____.

    ____ .The key to survive on planes is how quickly you can get out. So it's best to get seats as close as possible to a door. Or try to sit in the back of the plane because passengers in the back seats are safer than those in the front.

    ____ .First of all, read the safety information card and listen to the safety speech carefully. Every type of plane has different safety instructions. If you're sitting near a door, make sure you know how to open it if you need to. What's more, don't forget to keep your seat belt fastened(系牢) .

    ____.When there is a fire, breathing smoke in is one of the main causes of death. So cover your nose and mouth with a cloth to avoid it.

    ____.Don't be too nervous and try to keep a cool head all the time. Remember that even in the worst situation, you still have a chance to be safe.

下列化学反应属于中和反应的是（　　）

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—There will be more and more robots going into people's homes.

—        . We might be able to leave all the chores to them.