怎么让select 多选自动默认为全选?(急!!!)

程序如下:在没有对其进行选择的情况下,当提交时让他默认为全自动选择提交!!!谢谢各位啦!!!
<form name="form1">
<select name="sel1" size="4" multiple id="sel1" style="width:210px; height:120px ">
<option value="0">1</option>
<option value="1">2</option>
<option value="2">3</option>
<option value="3">4</option>
<option value="4">5</option>
<option value="6">6</option>
</select>
<input type="submit" onclick="add()">
</form>

<form name="form1">
<select name="sel1" size="4" multiple id="sel1" style="width:210px; height:120px ">
<option value="0,1,2,3,4,5,6" selected="selected">请选择(默认为全选)</option>
<option value="0">1</option>
<option value="1">2</option>
<option value="2">3</option>
<option value="3">4</option>
<option value="4">5</option>
<option value="6">6</option>
</select>
<input type="submit" onclick="add()">
</form>
其实这种用多选框checkbox实现是最合适的