Adding subelement in xml with integer value to a key

Question

I want to assign a new sub element to an existing xml using element tree.

below original xml -

<FILE>
<INSTANCE>
    <A>123</A>
    <B>ABC</B>
</INSTANCE>
<INSTANCE>
    <A>456</A>
    <B>DEF</B>
</INSTANCE>
</FILE>

Below expected xml -

<FILE>
<INSTANCE>
    <A>123</A>
    <B>ABC</B>
    <C>0</C>
</INSTANCE>
<INSTANCE>
    <A>456</A>
    <B>DEF</B>
    <C>0</C>
</INSTANCE>
</FILE>

I tried this -

import xml.etree.ElementTree as ET
tree = ET.parse("test.xml")
root = tree.getroot()
s= {"":"0"}
c = root.makeelement("C",s)
for child in root.findall('.//INSTANCE/A/..'):
    child.append(c)
tree.write("test1.xml")

i got below output -