单项选择题 Regulations require that cargo pumprooms on tank barges (constructed in 1965) carrying grades B or C liquid cargoes have power ventilation systems which can completely change the air in （）.

单项选择题 Petroleum cargo tanks should not be topped off at deck level when loading on a cold day because（）.

单项选择题 One of the principal dangers inherent in liquefied petroleum gas is（）.