在预防流行性出血热的措施中，下列哪项最重要（）
A."三早"预防
B.预防接种
C.灭螨防螨
D.灭鼠防鼠
E.疫情监测
参考答案：D
______ our arriving at the airport, we were given a warm welcome by the local people. [ ]
A. With
B. At
C. Upon
D. Until
半通行地沟内，管道或保温层外表面至沟壁的净距的取值范围为（）。
A.50～100mm
B.100～150mm
C.100～200mm
D.200～300mm
