无法使用XmlWriter编码为UTF8

我正在WCF中开发一个web服务。

我想返回一个UTF8格式的xml文件。

我的代码是:

StringBuilder output = new StringBuilder();
XmlWriterSettings settings = new XmlWriterSettings { Indent = true, Encoding = Encoding.UTF8 };
using (XmlWriter writer = XmlWriter.Create(output, settings))
{
    writer.WriteStartDocument();
    writer.WriteStartElement("Response", Resources.nameSpaceUri);
    /* Some functions */
    writer.WriteEndElement();
    writer.WriteEndDocument();
}
return output.ToString();

但结果是:

<?xml version="1.0" encoding="utf-16"?>
<Response xmlns="http://namespace">
    <!-- Content -->
</Response>

为什么我的回答是UTF-16?