A、B为同主族的两元素，A在B的上一周期，若A的原子序数为n，则B的原子序数不可能为
A.n+8 B.n+18 C.n+32 D.n+20
答案：D
How is the external flotation bladder of an immersion suit inflated?（）
A.It is inflated by a small CO2 bottle that is automatically tripped when the front zipper is at the top of the zipper track
B.It is inflated by a small CO2 bottle that is manually tripped
C.It is inflated by blowing through an inflation tube
D.It inflates by seawater bleeding into the inflation bladder and reacting with a chemical
汽车掉头应选择（）。
A.交叉路口.工厂门口；
B.广场.停车场；
C.道路较宽，交通流量较小的地方
