May we leave the classroom now? No, you _____. You_____ to leave until the bell rings.
A:mustn't, are allowed
B:don't have to , are supposed
C:needn't , aren't allowed
D:can't , aren't supposed
a、b两种物质的溶解度曲线如图所示,下列叙述正确的是( )
下列函数中,最小正周期为π,且图象关于直线x=π3对称的是
某实验小组在用天平和量筒测量液体的密度时,测量步骤为:①先测出空烧杯的质量;②将液体倒入烧杯,测出总质量;③算出烧杯中液体的质量;④将烧杯中的液体倒入量筒,测出其体积;⑤利用公式算出液体的密度.将对实验结果的分析正确的是( )
两极格局彻底瓦解的标志是( )