It is said that bad living habits as well as dirty surroundings, stuffy rooms and some other

factors _____ the wide spread of H1N1.   [ ]

A. result from      

B. benefit from    

C. devote to      

D. contribute to

组合间隙中间因为存在着一个容易放电的人体，所以，相同长度的组合间隙的放电电压比单一间隙的放电电压（）。也就是说，组合间隙的最小距离要求比单一间隙的最小距离（）。