A:①②
B:①③
C:②④
D:③④
—Is first impression very important?
—Yes, it is, because you never get second chance to make first impression.
—Hello, John! This is my friend, Tom.
— _____________________.
右图是反射弧的结构示意图,有关叙述错误的是
如图所示,在y轴上关于O点对称的A、B两点有等量同种点电荷+Q,在x轴上C点有点电荷-Q且CO=OD,∠ADO=60°.下列判断正确的是( )