用作标定滴定溶液浓度的试剂，其纯度应为（）
A.化学纯
B.分析纯
C.实验试剂
D.光谱纯
E.基准级纯或优级纯
参考答案：E
-—Are the two girls your cousins?
—_____. They are my sisters. [ ]
A. Yes, they are
B. No, they aren't
C. No, she isn't
D. Yes, she is
要使液压缸活塞的运动基本平稳，应采用（）。
A.节流阀
B.溢流阀
C.调速阀
D.顺序阀
