掘进巷道时的通风叫（）。其主要特点是：只有一个出口，本身不能形成通风系统。
参考答案：掘进通风
Not until he left his home _____ to know how important the family was for him.[ ]
A. did he begin
B. had he began
C. he began
D. he had begun
安全是免除了（）的损害风险的状态。
