55.There is supposed to be a safety ------ which makes it im

appliance的英文释义是“a device,machine or piece of equipment,esp。an electrical one that is used in the house such as a cooker,fire,washing machine,etc”强调的是具体的“设施,器械”,而machinery是对机械的泛称。此句中出现了对appliance的修饰语,已经说明了句子指的是具体事物。另外,“安全设施”如果用“safety mechanism”就太狭隘了,“设施”并非单指机械。Hope these can help you。