胎儿脊柱的正常声像图表现为:( )① 纵切面呈一条排列整齐的光带;② 纵切面呈串珠状的小光团排列成两条弓状平行光带;③ 纵切面呈一连串念珠状的小光团;④ 横切面可见3个强回声团,呈“品”形
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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