另外，如果免费用户中升级的比例低于预期，还可以通过广告进一步补充这个模型。
In addition, this model can be further supplemented with advertising if the ratio of free users to upgrades is less than expected.
即使操作系统/应用程序可以被升级，但这也不可能是 免费的。
Even in cases where the OS/applications can be upgraded, this is unlikely to be free of charge.
大份的免费项目包括过去十年的基础设施的升级。
The big free stuff includes much of the infrastructure upgrade of the past decade.
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