After the terrible earthquake, hundreds of people were ____ (home).
____
空气中氮气的体积分数大约是( )
如图,已知抛物线y=﹣x2+bx+c与x轴交于A(﹣1,0)、B(3,0)两点,与y轴交于点C,抛物线的对称轴与抛物线交于点P、与直线BC相交于点M,连接PB.
下列实验基本操作中,正确的是