如图,△ABC与△DEF是位似图形,位似比为2:3,已知AB=4,则DE的长为____
4的算术平方根是( )
人体有保卫自身的三道防线。下列免疫活动中属于特异性免疫的是( )
Xiao Li was supposed __________ her homework before nine o'clock. But she couldn't.