（4）事件处理——（9）让其他按钮可用（Enabling the other buttons）
We now have a Large Printbutton that works as advertised, but we need to apply similar handling to the other two buttons (Defaultand Narrow Column) to make them perform their tasks. This is straightforward; we use .bind()to add a clickhandler to each of them, removing and adding classes as necessary. The new code reads as shown in the following code snippet:
Clicking on Defaultremoves both class names from the <body>tag, returning the page to its initial rendering.
现在点击窄行间距按钮后，对应的css将会被应用，然后文本将会变得不同：
点击默认按钮，将移去body标签上的两个类，染回网页初始渲染的样子。
