问题补充：
单选题Ifthereisanintruder_____thecampus,studentsshouldbe_____throughanannouncement,______teachersshouldlocktheirclassroomstokeepthechildrensafe.A.in, warned, after thatB.at, warned, before whichC.on, informed, after whichD.in, informed, before which
答案：
C
