Goldfish don't know _______ to them three seconds ago.
A:what happens
B:what is happening
C:what happen
D:what happened
B)根据句意及括号内所给单词的提示填空。
将烧瓶内的水加热至沸腾后移去火焰,水会停止沸腾.迅速塞上瓶塞,把烧瓶倒置并向瓶底浇冷水(如图),你会观察到烧瓶内的水又沸腾起来,产生这一现象的原因是( )
大雁的飞行器官是( )