A:How was your holiday?
B:When was your holiday?
C:Where was your holiday?
D:What was your holiday?
小明开着锅盖煮鸡蛋.第一次水烧开后继续用急火加热;第二次水烧开后,将火焰调小,但仍保持锅中的水沸腾.则( )
化简:(a﹣2a-1a)÷a2-1a= ____
胚盘是鸟卵发育的起始部位.
地球自转和公转运动特征中,相同的是( )