The buttons do work initially but if you change the selected option only the first selected options is passed when the form is submitted.
按钮做的初步工作，但如果你改变选定的选项只有第一个选定的选项是通过提交表单时。
Make sure the first option is selected and then click on Add.
选择第一项，然后点击添加。
When performed at development time, this process is very similar to the first option listed above in which we explicitly selected the Add class' implementation of the Math interface.
在开发时执行时，此过程与上面列出的第一个选项非常相似，我们在此选择中显式地选择 Add 类的 Math 接口实现。
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