A:lose
B:will be lost
C:are lost
D:will lose
The writer mentions birds and frogs to .
如果单项式xa+1y3与2x3yb是同类项,则a,b的值分别为____.
下列叙述中,符合经线特点的是( )①与地面垂直 ②长度不等 ③与纬线垂直相交 ④连接南、北两极