在生命系统的各个层次中，能完整的表现出各种生命活动的最微小的层次是 （ ） A．分子原子层次 B．细胞层次 C．种群和群落层次
A young officer was teaching some old soldiers. They had been in the army for many years and did not like officers, young or old. They did not think this young officer could tell them anything about h
x取下列各数中的哪个数时，二次根式有意义（ ） A．﹣2B．0C．2D．4
下列实验不能成功的是( ) A．用肥皂水检验软水和硬水 B．用食醋区分食盐和小苏打 C．将冷碟子放在蜡烛火焰的上方，得到炭黑 D．加熟石灰粉末研磨
— Where _____? — I got stuck in the heavy traffic. I _____ here earlier. A. did you go； had arrived B. have you been； would have been C. were you； would come
关于原点对称的点的坐标为（ ）
B．
C．
D．
关于原点对称的点的坐标为