First of all we need references.
首先我们需要参考图。
The reason is that we need references to both ProgressBars as arguments to the callback, and at the time the Scale constructor is invoked, they are not all defined yet.
原因在于，我们需要以对两个ProgressBar的引用作为回调的参数，而当Scale构造函数被调用时，它们还没有被定义。
We need at least ten references for the essay.
那份论文我们需要至少10本参考书目。
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