Some damage has been caused u____ by people simply walking around the site and some has been caused deliberately.
免疫对人体既有保护作用,也有不利因素.(判断对错)
单词拼写:根据首字母或汉语意思,写出正确的单词。
学好科学知识,培养保卫和谐家园的本领.已知某定时炸弹,其引爆装置如图所示,起爆前定时开关S是闭合的,当设定起爆时间一到,定时开关S会自动断开,为使引爆装置停止工作,下列操作和过程分析正确的是( )