在竖井内如空气温度高于室外，空气的密度随之出现差别，由于浮力作用将导致烟囱效应。（）
参考答案：对
常用的无损检测方法有：超声波、（ ）、X射线、磁粉
A．着色
B．放大镜观察
C．焊缝规检查
Not until he left his home ________ to know how important the family was for him.[ ]
A. did he begin
B. had he begun
C. he began
D. he had begun
