—Guess what? I happened to have met Mark at the party last week.
—If my memory serves me correctly, you two ________ each other for almost ten years.
A:haven't seen
B:hadn't seen
C:didn't see
D:don't see
收集氧气时检验集气瓶内氧气是否收集满,最好的方法是 ( )
某市博物馆利用双重防盗系统来加强对名贵古董的保护,此报警系统的核心部分包括两个开关和一个报警铃.只要有人靠近展台,就会触发隐藏在展台四周踏板下面的开关S1(相当于闭合开关S1),报警铃就会响起;只要将古董取走,无论人是否靠近展台,都会触发隐藏在古董底部的开关(相当于闭合开关S2),报警铃也会响起.请你帮助小明设计出此防盗系统的电路图.