面向对象：一个金融系统的项目进度和资源 求解答?【google面试题】

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        When you are counting key classes identified in analysis to estimate a financial system's projects schedule and resources, which one of the following could you use to define a key class?
Choice 1
        Classes not being reused from previous projects
Choice 2
        Classes not purchased from a third party
Choice 3
        Classes that model "business" behavior
Choice 4
        Classes that make up a user interface
Choice 5
        Classes that interface directly with the database