这个联动如何实现

<form name="Search" action="nk.asp" method="post">
<select name="dz1" id="dz1" size="1" style="width:100; height:19" onchange="chan(this)">
<option value="A" selected>A</option>
<option value="B">B</option>
<option value="C">C</option>
<option value="D">D</option>
<option value="E">E</option>
<option value="F">F</option>
</select>

<select name="dz2" id="dz2" size="1" style="width:100; height:19">
<option value="A" selected>A</option>
<option value="B">B</option>
<option value="C">C</option>
<option value="D">D</option>
<option value=&