<problem>
  <choiceresponse partial_credit="halves">
    <label>Which of the following is a fruit?</label>
    <description>Select all that apply.</description>
    <checkboxgroup>
      <choice correct="true">apple</choice>
      <choice correct="true">pumpkin</choice>
      <choice correct="false">potato</choice>
      <choice correct="true">tomato</choice>
    </checkboxgroup>
  </choiceresponse>
</problem>