How long should we keep the video tapes of surveillance cameras？
A. You should keep the video tapes for at least five months.
B. The video tapes should be kept for at least three months.
C. You should keep the video tapes for at least one month
D. You should keep the video tapes for at least two months.
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等离子弧切割基本原理是利用等子弧把被切割的材料局部()，并同时用高速气流吹走。
A. 熔化及蒸发
B. 氧化和燃烧
C. 熔化和燃烧
D. 燃烧及蒸发
速度为100～200km/h正线道岔轨道静态临时补修轨距容许偏差管理值为()mm。
道岔导曲线下股高于上股限值,临时补修管理值为()。
并联电阻越多，其总电阻值().
A. 不变
B. 越大
C. 越小
D. 近似
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