已知R2+离子核外有a个电子，b个中子。表示R原子符号正确的是 A． B． C． D．
My grandmother was an iron-willed woman, the feared head of the family. When I was five years old, she invited some friends to her apartment for a party. 26the guests was a neighborhood big shot(大
A good friend of mine from ______ I was born showed up at my home right before I left for Beijing. A. how B. whom C. when D. whi
种子贮存时可采取密封保存或低温保存的方法。下列分析中不正确的是（ ） A．密封保存和低温保存都能抑制种子内有机物的分解 B．密封保存可造成
20如图1-2-24所示，甲、乙两辆同型号的轿车，它们外形尺寸如下表所示．正在通过十字路口的甲车正常匀速行驶，车速=10 m／s，车头距中心O的距离为20
“α＝”是“cos2α＝”的( ) A．充分不必要条件B．必要不充分条件C．充分必要条件 D．既不充分也不必要条件
的最小正周期为 （ ）
B．
C．
D．
