急，关于动态修改下拉菜单

急，关于动态修改下拉菜单内容<form name="form1"><select name="select1"><option value="a">a</option><option value="b">b</option><option value="c">c</option><option value="d">d</option></select></form>请问怎样让他的内容变成<option value="e">e</option><option value="f">f</option><option value="g">g</option>谢谢!<script>document.onclick=change_option;function change_option(){var o=form1.select1;o.options.length=3;o.options[0].value="e";o.options[0].text="e";o.options[1].value="f";o.options[1].text="f";o.options[2].value="g";o.options[2].text="g";}</script><form name="form1"><select name="select1"><option value="a">a</option><option value="b">b</option><option value="c">c</option><option value="d">d</option></select></form>点击任意地方试试