题目

Better be more careful______you've made a mistake.          A．when                             B．where                           C．and                                D．that 答案：B你观察过养在鱼缸中的鱼吗?它的口和鳃盖后缘，不断地相互交替张开和关闭。这是鱼在进行哪种生理活动A．取食 B．呼吸 C．排泄 D．生殖