C#将DataTable循环遍历之后拼接为xml结构时提示“有多个根节点”

if (dt != null && dt.Rows.Count > 0)
{
for (int i = 0; i < dt.Rows.Count; i++)
{
XDocument myDoc = new XDocument
(
new XElement("Response",
new XElement("Item", new XElement("PATIENT_ID", dt.Rows[i]["PATIENT_ID"]),
new XElement("VISIT_ID", dt.Rows[i]["VISIT_ID"]),
new XElement("INP_NO", dt.Rows[i]["INP_NO"]),
new XElement("NAME", dt.Rows[i]["NAME"]),
new XElement("SEX", dt.Rows[i]["SEX"]),
new XElement("DATE_OF_BIRTH", dt.Rows[i]["DATE_OF_BIRTH"]),
new XElement("ADMISSION_DATE_TIME", dt.Rows[i]["ADMISSION_DATE_TIME"]),
new XElement("DEPT_ADMISSION_TO", dt.Rows[i]["DEPT_ADMISSION_TO"]),
new XElement("DISCHARGE_DATE_TIME", dt.Rows[i]["DISCHARGE_DATE_TIME"]),
new XElement("DEPT_DISCHARGE_FROM", dt.Rows[i]["DEPT_DISCHARGE_FROM"]),
new XElement("DISCHARGE_TYPE", dt.Rows[i]["DISCHARGE_TYPE"]),
new XElement("TOTAL_COST", dt.Rows[i]["TOTAL_COST"]),
new XElement("CONSULTING_DOCTOR", dt.Rows[i]["CONSULTING_DOCTOR"]),
new XElement("ATTENDING_DOCTOR", dt.Rows[i]["ATTENDING_DOCTOR"]),
new XElement("DOCTOR_IN_CHARGE", dt.Rows[i]["DOCTOR_IN_CHARGE"]),
new XElement("DIRECTOR", dt.Rows[i]["DIRECTOR"]),
new XElement("MAILING_ADDRESS", dt.Rows[i]["MAILING_ADDRESS"]),
new XElement("NEXT_OF_KIN", dt.Rows[i]["NEXT_OF_KIN"]),
new XElement("NEXT_OF_KIN_ADDR", dt.Rows[i]["NEXT_OF_KIN_ADDR"]),
new XElement("NEXT_OF_KIN_PHONE", dt.Rows[i]["NEXT_OF_KIN_PHONE"]),
new XElement("HOSPITAL_STATUS", dt.Rows[i]["HOSPITAL_STATUS"]),
new XElement("PATIENT_ROLE", dt.Rows[i]["PATIENT_ROLE"]),
new XElement("DISEASE_OUTCOME", dt.Rows[i]["DISEASE_OUTCOME"])))
);
ResponseStr += myDoc.ToString() + Environment.NewLine;
}
ResponseStr = new XDeclaration("1.0", "UTF-8", null) + Environment.NewLine + ResponseStr;
xd.LoadXml(ResponseStr);

xml只能有一个根节点，比如说
<a>xxx</a>
<b>xxx</b>
上面这个是不可以的。
而
<root>
<a>xxx</a>
<b>xxx</b>
</root>
这是可以的。