下拉框联动文本框怎么做？（在线等待）

Question

<script>
function show(value){ 
var head = document.getElementById("aa"); 
head.innerHTML=value;
}
</script>
<table>
<tr>
        <td>结算类型：</td><td><select name="types" onchange="show(this.value)">
                <option value="1" >及时结算</option>
                <option value="2" >按天结算</option>
                <option value="3" >按金额结算</option>
                </select>
        </td>         
        </tr>
        <tr>
        <td id="aa">AAA </td>要改变的地方
                <td></td>
        </tr>
</table>

Answer 1

<script language="javascript">
function aa()
{
if(document.getElementById("types").value=="1")
{
document.getElementById("a").innerHTML = "bbb";
}
if(document.getElementById("types").value=="2")
{
document.getElementById("a").innerHTML = "ccc";
}
if(document.getElementById("types").value=="2")
{
document.getElementById("a").innerHTML = "ddd";
}
}
</script>
<tr>
        <td>结算类型：</td><td>
        <select name="types" onChange="aa()" >
                <option value="1" >及时结算</option>
                <option value="2" >按天结算</option>
                <option value="3" >按金额结算</option>
                </select>
        </td>         
        </tr>
        <tr>
        <td><div id="a">AAA</div> </td>要改变的地方
                <td></td>
        </tr>
没测试

Answer 2

<script>
function show(value){ 
var head = document.getElementById("aa"); 
head.innerHTML=value;
}var head = document.getElementById("aa"); 
是什么意思呀，还有head.innerHTML=value，看不懂呀；

Answer 3

function Change()
{
var str = document.getElementById("types").value;
switch(str)
{
case "及时结算":
document.getElementById("td").innerText="AAA";
break;
case "按天结算":
document.getElementById("td").innerText="BBB";
break;
case "按金额结算":
document.getElementById("td").innerText="CCC";
break;
}
}
<tr>
        <td>结算类型：</td><td><select name="types" >
                <option value="1" >及时结算</option>
                <option value="2" >按天结算</option>
                <option value="3" >按金额结算</option>
                </select>
        </td>         
        </tr>
        <tr>
        <td id="td">AAA </td>要改变的地方
                <td></td>
        </tr>

Answer 4

function Change()
        {
            var str = document.getElementById("types").value;
            switch(str)
            {
                case "1":
                    document.getElementById("td").innerText="AAA";
                    break;
                case "2":
                    document.getElementById("td").innerText="BBB";
                    break;
                case "3":
                    document.getElementById("td").innerText="CCC";
                    break;
            }
        }

Answer 5

<script>
function show(value){ 
var head = document.getElementById("aa"); 
var str;
switch(value){
case "1":
str="bbb";
break; 
case "2":
str="ccc";
break; 
case "3":
str="ddd";
break; 
default:
str="AAA";
}
 head.innerHTML=str;
}
</script>
<table>
<tr>
        <td>结算类型：</td><td><select name="types" onchange="show(this.value)">
                <option value="1" >及时结算</option>
                <option value="2" >按天结算</option>
                <option value="3" >按金额结算</option>
                </select>
        </td>         
        </tr>
        <tr>
        <td id="aa">AAA </td>要改变的地方
                <td></td>
        </tr>
</table>