I need an XPATH expression to to the following

I have a DOM as below

<Data>
<Object>
<A ExternalId="A"/>
<B ExternalId="B"/>
<A ExternalId="C"/>
<B ExternalId="D"/>
</Objects>
</Data>


I am looking for an XPath expression that will give me the node
<A ExternalId="C"/>

However all I know is that the node is a first level child of "/Data/Objects" and that it has an attribute "ExternalId" with a value of "C"

I tried the following but it doesnt work

oDOM.selectSingleNode("/Data/Objects/.[@ExternalId='C']")

Any ideas