单项选择题 A number of biological sewage treatment plants are in use at sea but nearly all work on what is called the extended aeration processBasically the consists of oxygenating the liquor by bubbling air through itAccording to the above two sentences, the word "this" means （）.

单项选择题 Explosions do occur （） caused by the ignition of oil or vapour gaining access from compressors or defective engine starting valves.

单项选择题 Air reservoirs are designed, built and tested similarly to （）.