生产率的定义是（）。
A．一切经济价值的源泉
B．产出与投入之比
C．衡量生产系统转换效率
D．生产要素（资源）的使用效率
参考答案：B
假如没有电流的磁效应，下列场景合理的是[ ]
A、电磁起重机不能搬运钢铁
B、灯泡可以发光
C、不能进行温度自动控制
D、电铃不能发声了
说明理由：
One tool that is useful during both analysis and design is the ______, which is a pictorial representation of the items of information(entities) within the system and the relationships between these pieces of information.
A．data dictionary
B．dataflow diagram
C．use case diagram
D．entity-relationship diagram
