灭火战斗中，公安消防部队的组织指挥原则是（）。
A.救人第一
B.先控制、后消灭
C.集中兵力打歼灭战
D.统一指挥，逐级指挥
参考答案：D
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
粒子的衍射是由光粒子间的相互作用引起。
