采气树每（）小时进行一次巡查，观察各气井采气树有无泄漏、抖动、腐蚀点、压力变化等异常现象。
A.3
B.4
C.5
D.6
参考答案：B
孟德尔对遗传规律的探索经过 [ ]
A．分析→假说→实验→验证
B．实验→分析→假设→结论
C．假设→实验→结论→验证
D．实验→假设→验证→结论
Listen! His family must be quarrelling, ().
A. mustn't it
B. isn't it
C. aren't they
D. needn't they
