<form name="form1" method="post" action="">
<p>
<select name="select" onchange="this.form.select2.selectedIndex=this.selectedIndex">
<option value="1">1</option>
<option value="2">2</option>
</select>
</p>
<p>
<select name="select2">
<option value="yes">yes</option>
<option value="no">no</option>
</select>
</p>
</form>
<p>
<select name="select" onchange="this.form.select2.selectedIndex=this.selectedIndex">
<option value="1">1</option>
<option value="2">2</option>
</select>
</p>
<p>
<select name="select2">
<option value="yes">yes</option>
<option value="no">no</option>
</select>
</p>
</form>
<form name="form1" method="post" action="">
<p>
<select name="select">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</p>
<p>
<select name="select2">
<option value="yes">yes</option>
<option value="no">no</option>
</select>
</p>
</form>select1 选择3的时候,select2选择为yes,其他均为选择为no
<p>
<select name="select" onchange="this.form.select2.selectedIndex=this.value=='3'?0:1">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</p>
<p>
<select name="select2">
<option value="yes">yes</option>
<option value="no" selected>no</option>
</select>
</p>
</form>
<p>
<select name="select" onchange="this.form.select2.selectedIndex=(this.selectedIndex==2)?0:1">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</p>
<p>
<select name="select2">
<option value="yes">yes</option>
<option value="no">no</option>
</select>
</p>
</form>