How to write this XQuery program?

Hi, all:

Now I have three XML files, c1.xml, c2.xml, c3.xml.
Each XML file contains information of one company, and of parts supplied by it.
Given a part id, I want to get the company which offered this part with the lowest price.
How can I write this XQuery?

-----begin of source data----
c1.xml:
 <company>
  <name>CompanyA</name>
  <parts>
   <part>
    <id>p1</id>
    <price>100</price>
   </part>
   <part>
    <id>p2</id>
    <price>101</price>
   </part>
  </parts>
 </company>
c2.xml:
 <company>
  <name>CompanyB</name>
  <parts>
   <part>
    <id>p2</id>
    <price>105</price>
   </part>
   <part>
    <id>p3</id>
    <price>106</price>
   </part>
  </parts>
 </company>
c3.xml:
 <company>
  <name>CompanyC</name>
  <parts>
   <part>
    <id>p2</id>
    <price>100</price>
   </part>
   <part>
    <id>p6</id>
    <price>106</price>
   </part>
  </parts>
 </company>
-----end of source data----

-----begin of expected result----
<result>
 <part>
  <id>p2</id>
  <price>100</price>
  <company>CompanyC</company>
 </part>
</result>
-----end of expected result----

Who would like to give me a hand? Thanks very much.

best regards,
-zhimao