Search for XML elements using LINQ

I have the following XML file :

<warehouse>
          <cat id="computer">
            <item>
              <SN>1</SN>
              <name>Toshiba</name>
              <quantity>12</quantity>
              <description>CPU: CORE I5 RAM: 3 GB HD: 512 GB</description>
              <price>400 USD</price>
            </item>
<item>
              <SN>22</SN>
              <name>Toshiba</name>
              <quantity>12</quantity>
              <description>CPU: CORE I5 RAM: 3 GB HD: 512 GB</description>
              <price>400 USD</price>
            </item>
          </cat>
          <cat id="Stationery">
            <item>
              <SN> 33 </SN>
              <name>note books</name>
              <quantity>250</quantity>
              <description>Caterpiller</description>
              <price>5 USD</price>
            </item>
        </cat>
        <cat id="Furniture">
            <item>
              <SN> 1 </SN>
              <name>dasd</name>
              <quantity>asdasd</quantity>
              <description>das</description>
              <price>dasd</price>
            </item>
<item>
              <SN>44</SN>
              <name>Toshiba</name>
              <quantity>12</quantity>
              <description>CPU: CORE I5 RAM: 3 GB HD: 512 GB</description>
              <price>400 USD</price>
            </item>

            <item> 
        </cat>
        </warehouse>

and i have the following web form page for generate queries :(i need reputation to insert image so i'll try to descrip it)

choose where to search : (dropdownlist)
insert sn : (textbox)
generate query: (button)
[labels]

I want to return (name,description,price,quantity) for SN==Textbox.text where <cat id= dropdownlist.text> in the label. can u help using LINQ or XPATH or any other methods.

I have tried this code , but i dont know to insert the SN==Textbox.text where <cat id= dropdownlist.text> condition into it !! please help me.

 XDocument xmlDoc = XDocument.Load(Server.MapPath("/XML/Cat1.xml"));
    var persons = from person in xmlDoc.Descendants("item")

    select new
    {
    SN = person.Element("SN").Value,
    name = person.Element("name").Value,
    quantity = person.Element("quantity").Value,
    description = person.Element("description").Value,
    price = person.Element("price").Value,
    };

    foreach (var person in persons)
{
label1.Text = person.SN "<br />";
label2.Text = person.name"<br />";
label3.Text = person.description"<br />";
label4.Text = person.price"<br />";
label5.Text = person.quantity"<br />";
}