A:was playing
B:played
C:has played
D:had played
“人间四月芳菲尽,山寺桃花始盛开”,造成这一差异的环境因素是( )
课外探究时,某兴趣小组同学选择了“金属和酸反应快慢与什么因素有关”课题.
已知函数f(x)=ax2+2x+1,-2<x≤0ax-3,x>0有3个零点,则实数a的取值范围是 ____.