某人要到60千米外的农场去,开始他以5km/h的速度步行,后来又以速度是18km/h的拖拉机把他送到农场,总共用了5.5h,他步行了
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
免费的网站请分享给朋友吧