Add form to table rows

Which is a valid way (if any) to add a form to table rows?

I have the following situation:

<table>
  <tr>
    <td><input type="text" name="q"></td>
    <td><input type="text" name="a"></td>
    <td><input type="submit" name="submit" value="Submit"></td>
  </tr>
  <tr>
    <td><input type="text" name="q"></td>
    <td><input type="text" name="a"></td>
    <td><input type="submit" name="submit" value="Submit"></td>
  </tr>
  <tr>
    <td><input type="text" name="q"></td>
    <td><input type="text" name="a"></td>
    <td><input type="submit" name="submit" value="Submit"></td>
  </tr>
</table>

How can I add a form element and still have valid HTML?

<table>
  <form>
    <tr>
      <td><input type="text" name="q"></td>
      <td><input type="text" name="a"></td>
      <td><input type="submit" name="submit" value="Submit"></td>
    </tr>
  </form>
</table>

Is invalid (at least I think it is)