XQuery and Value method and syntax

What should the proper syntax be to pull the data back where value2 is yo5?

SELECT AdditionalData.value('
declare namespace C="test";
(/C:DocumentElement/C:AdditionalData/C:Value2)[1]', 'VARCHAR(12)') AS answer
from table

I am trying to pull data from an xml data type column. The column when double clicked on looks like:

<DocumentElement>
<AdditionalData>
<AdditionalDataID>1</AdditionalDataID>
<DataCategoryID>1a</DataCategoryID>
<Value1>yo1</Value1>
<Value2>yo2</Value2>
<Value3>yo3</Value3>
</AdditionalData>
<AdditionalData>
<AdditionalDataID>2</AdditionalDataID>
<DataCategoryID>2a</DataCategoryID>
<Value1>yo4</Value1>
<Value2>yo5</Value2>
<Value3>yo6</Value3>
</AdditionalData>
</DocumentElement>