如何在XML文档中包含&符号(&)？

我怎么能把& XML标记的属性中的符号，同时保持XML有效？

<?xml version="1.0" ?> 
<a text="b&c"/>

当我使用W3School’s validator，我得到错误：

EntityRef: expecting ‘;‘

使用字符参考来表示它：& amp; amp; amp;

见specification：

The ampersand character (&) and the left angle bracket (<) MUST NOT appear in their literal form,except when used as markup delimiters,or within a comment,a processing instruction,or a CDATA section.

If they are needed elsewhere,they MUST be escaped using either numeric character references or the strings ”
&
” and ”
<
” respectively. The right angle bracket (>) may be represented using the string ”
>
“,and MUST,for compatibility,be escaped using either ”
>
” or a character reference when it appears in the string ”
]]> ” in content,when that string is not marking the end of a CDATA section.