我们传递给Substring一个参数：我们想要开始提取字符的开始位置。

We pass Substring a single parameter: the starting position where we want to begin extracting characters.

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如果指定了可选的起始位置，则表明它是源字符串中开始进行搜索的字符位置。

If the optional start is specified, it indicates the character position in the source-string at which the search is to begin.

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第一，在创建字符串时，您可以混合使用单引号和双引号，只要字符串在开始位置和结束位置使用同一类型的引号。

First, you can mix single and double quotation marks when creating a string, as long as the string USES the same type of quotation mark at the beginning and end.

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