创建单选钮控件。
Creates a radio button control.
使用单选钮可以将用户可选择的范围限制到一组值中的一个。要实现这一功能，你必须为同一组的单选钮取相同的 name 值。
当用户提交表单时，只有选中的单选钮才会提交其 name / value 的对值，如果该单选钮有值的话。
要将一组单选钮中的一个设置为默认按钮，请将该单选钮的 checked 属性设置为 true。
只有当单选钮指定了 name 属性时，用户才能够选择该单选钮。要取消对某单选钮的选中，用户必须选择该单选钮组中的另外一个。
此元素在 Internet Explorer 3.0 及以上版本的 HTML 和脚本中可用。
此元素是一个内嵌元素。
此元素不需要关闭标签。
Use a radio button control to limit a user's selection to a single value within a set of values. To do this, you must link together each button in a set of radio buttons by assigning each button the same name.
When a user submits a form, a selected radio button only generates a name/value pair in the form data if the button has a value.
To select a radio button as the default button in a set, set the checked property of the button to true.
A user can select a radio button only if the button has a name. To clear a selected radio button, a user must select another button in the set.
The INPUT type=radio element is available in HTML and script as of Internet Explorer 3.0.
This element is an inline element.
This element does not require a closing tag.
<input type="radio" name="fruit" value = "Apple">苹果<br>
<input type="radio" name="fruit" value = "Orange" checked>桔子<br>
<input type="radio" name="fruit" value = "Mango">芒果<br>