北京和呼和浩特相距660千米.一列火车从呼和浩特开出，每时行驶48千米;另一列火车从北京开出，每时行驶72千米.两列火车同时开出，相向而行，经过几时相遇?两车相遇时，从北京开出的火车比从呼和浩特开出的火车多行了多少千米?

如图所示，一竖直绝缘轻弹簧的下端固定在地面上，上端连接一带正电小球P，小球所处的空间存在着竖直向上的匀强电场，小球平衡时，弹簧恰好处于原长状态：现给小球一竖下向上的初速度，在小球从开始运动至达到最高点的过程中，以下说法正确的是

swim_______(现在分词)

阅读理解，根据短文内容判断正(T)误(F)。

It'sSundaymorning.ThestudentsofClass3aregivingtheirclassroomagoodcleaning.

MissHuang,theirteacher,isworkingwiththem.Thechildrenarebusy.Somearecarryingwater;

somearecleaningthewindows;othersaresweepingthefloor.ZhangHuaisputtingupamaponthe

wall.ItisamapofChina.WangFeiandWeiQingaremendingsomebrokenchairs.Thechildrenare.

listeningtotheradiowhiletheyareworking.Theclassroomlooksniceandbrightafterthecleaning.

Thechildrenareveryhappy.Theygohomeforlunchatnoon.

()1.ThechildrenareplayingintheirclassroomonSundaymorning.

()2.MissHuang,theirmother,isworkingwiththem.

()3.ThereisamapofChinaonthewall.

()4.Twoofthemarerepairingthebrokenchairs.

()5.Theyaresingingwhiletheyareworking.

足球比赛时，裁判用抛硬币猜正反面的方法让双方决定谁先要场地，谁先要发球权.这种做法是________的，因为________.