单项选择题
The airborne concentrations of substances (such as H2S) under which nearly all workers may be repeatedly exposed without adverse effects are called（）.
A.exposure limits B.concentration limits C.threshold limit values D.substance limit values
单项选择题 Storage batteries should be charged in a well ventilated area because（）.
单项选择题 One of the principal dangers inherent in liquefied petroleum gas is（）.
单项选择题 Name one major advantage of transporting gas under refrigeration.（）
