元朝的建立时间和建立者分别是( )
A:1271年,成吉思汗
B:1276年,忽必烈
C:1271年,忽必烈
D:1276年,铁木真
求证:四边形AECF是平行四边形.
读一读,选一选
____ Is your father here today? A. No, I don't.
____ What does your mother do? B. He's a businessman.
____ Do you want to be a head teacher? C. No. He's in Australia.
____ What does he do? D. She's a head teacher.
一元二次方程的x2+6x-5=0左边配成完全平方式后所得的方程为 ( )
利用生物反应器来生产某种物质,其意义不包括 ( )