xsl:key() and $variables.

<Records>
  <Record>
     <activity>Baseball</
     <hours>36</
  <Record>
     <activity>Shopping</
     <hours>39</
  <Record>
     <activity>Writing</
     <hours>3</
   ......etc  (yes, I know they're not closed.)

<activities>
   <activity neccesary="no">Baseball</
   <activity neccesary="no">Shopping</
   <activity neccesary="yes">Writing</

How do I  <xsl:apply-templates select="$variable/Records/Record[1]" />
only those records which are neccesary="yes"  ?