①双曲线 $\frac{x^2}{16}-\frac{y^2}{9}=1$ 与椭圆 $\frac{x^2}{49}+\frac{y^2}{24}=1$ 有相同的焦点；

②以抛物线的焦点弦（过焦点的直线截抛物线所得的线段）为直径的圆与抛物线的准线是相切的；

③设A，B为两个定点，k为常数，若|PA|﹣|PB|=k，则动点P的轨迹为双曲线；

④过定圆C上一点A作圆的动弦AB，O为原点，若 $\overrightarrow{OP}=\frac{1}{2}\left(\overrightarrow{OA}+\overrightarrow{OB}\right)$ 则动点P的轨迹为椭圆．其中正确的个数是（   ）

某足协举办了一次足球比赛,记分规则是:胜一场积3分,平一场积1分,负一场积0分.若甲队比赛了5场共积8分,则甲队可能平了(       )
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B

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如果细胞核内存在DNA解旋的现象，则此时细胞不可能处于（  ）

如图所示，已知一条入射光线过凸透镜的光心O，一条经过凸透镜折射后的光线与主光轴平行，请分别画出它们对应的出射光线和入射光线．

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