镜头的种类:1从焦距的变化上可分为:定焦和变焦镜头;2从对对焦方法可分为:手动对焦与自动对焦镜头;3按焦距的长短可分为:标准镜头、广角镜头、长焦镜头
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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