锅炉发生爆炸的原因有（）和（）等。
参考答案：超压、过热、腐蚀、槽裂、设计制造欠缺；水锤
They have never been to Beijing, _____? [ ]
A. have they
B. haven't they
C. do they
D. don't they
应答器日常维护有哪些主要内容？
