单项选择题
On a hydrocarbon flammability chart the line which extends from 0% to 21.8% oxygen，lying tangent to the flammability range，is called the（）.
A.mimimum oxygen content line B.critical displacement line C.critical dilution line D.upper threshold limit
单项选择题 不同类型的导管分子次生壁加厚的程度不同，下列哪一种排列正确的表明了壁逐渐增厚的程度？（）
单项选择题 One precaution to take prior to working in a freshly painted compartment where there are an excessive amount of paint fumes, is to （）.
单项选择题 将一个快捷方式图标删除后，（）。
