乳剂的不稳定现象有（）
A.分层
B.絮凝
C.转相
D.破裂
E.酸败
参考答案：A, B, C, D, E
设；则和的夹角θ为（）．
Jane will have to________her piano practice a long time if she wants to be a professional concert
performer.[ ]
A. set up
B. add up
C. keep up
D. hold up
；则
和
的夹角θ为（）．