<select name="select" onchange="if(this.options[this.selectedIndex].value!=''){location='http://www.baidu.com'}">
<option value="1" selected>1</option>
<option value="2" >2</option>
<option value="3" >3</option>
</select>
<option value="1" selected>1</option>
<option value="2" >2</option>
<option value="3" >3</option>
</select>
<select name="select" onchange="window.location=this.value;">
<option value="www.zzz.com" selected>1 </option>
<option value="www.yyy.com" >2 </option>
<option value="www.xxx.com" >3 </option>
</select>