离心泵出水压力的高低与叶轮的直径的大小和（）的高低有着直接的关系。
参考答案：叶轮转速
There ______ two supermarkets in the streets now. [ ]
A. is
B. are
C. was
D. were
药事管理研究有规范性、结合性、开放性、实用性等特征。（）
