Javascript to add a button to the next row in a table

I am trying to get the add button that appears on the next line of each line when I click the add button. I am currently getting an empty text box in the cell where I would like the add button to appear. What do I need to change in my code to get the Add button instead of the text field when I click the Add button?

For the add JavaScript button, I currently have:

     // button cell
  var cellRightSel = row.insertCell(3);
  var bt = document.createElement('input');
  bt.name = 'addBt' + iteration;
  bt.id = 'addBt' + iteration; 
  cellRightSel.appendChild(bt);

and html for the page:

<table border="1" id="tblSample">
<tr>
<th colspan="3">Sample table</th>
</tr>
<tr>

<td><input type="text" name="txtRow1"
 id="txtRow1" size="40"  /></td>



<td><input type="text" name="txtRow2"
 id="txtRow3" size="40"  /></td>



<td>
<select name="selRow0">
<option value="value0">text zero</option>
<option value="value1">text one</option>
</select>
</td>


<td><input type="button" value="Add" onclick="addRowToTable();" name="addBt0" /></td>


</tr>
</table>