站着不动、长期或经常重复地弯腰、躯干扭曲并倾斜的姿势、经常性重复地单腿支撑的姿势和手臂长时间前伸等姿势极易产生疲劳。其原因主要是( )。
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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