A:①②
B:②③
C:①④
D:③④
The students their pens when the teacher told them to stop writing.
化石证明,有许多物种只存活了相当短暂的时代就逐渐灭绝了,最能解释这一现象的是( )
如图甲所示,弹簧测力计下挂着一个实心圆柱体,将它逐渐浸入装有水的烧杯中,弹簧测力计的示数,随圆柱体下表面浸入的深度h的变化如图乙所示。下列结论错误的是(g取10N/kg) ( )