While ____ the dog, you were careless and it got ____ and was hit by a car.
如图所示,在京昆高速公路266km处安装了一台500万像素的固定雷达测速仪,可以精准抓拍车辆超速,以及测量运动过程中车辆的加速度.若B为测速仪,A为汽车,两者相距355m,此时刻B发出超声波,同时A由于紧急情况而刹车,当B接收到反射回来的超声波信号时,A恰好停止,且此时A、B相距335m,已知声速为340m/s,则汽车刹车前的正常行驶速度大小为( )
Research shows that people feel unhappy and worried when they have nothing to do.________,the happiest are those who are busy.
公元前4世纪,亚历山大建立起地跨欧、亚、非三洲的大帝国,其首都设在( )