地球上以半个朔望月为周期变化的海洋潮汐为（）
A.涨潮与落潮
B.高潮于低潮
C.大潮与小潮
D.潮差
参考答案：C, D
Water is a liquid which has no color _______ you wash your hands in it, then it turns black.[ ]
A. before
B. until
C. since
D. because
EMT-31a电磁断路器
