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According to the lecture, what is "bartering"

A．Trying to get something for a cheaper price.

B．Exchanging one product or service for another.

C．Trading products in an unstable economy.

D．Specializing in the production of one goo

UML的（）模型图由类图、对象图、包图、组件图、和部署图组成。

A.用例

B.静态

C.动态

D.系统