If there are any spelling mistakes, I am german. 

The user of my site should say, in which class of school he is.
Since it is possible that he doesn't go to school any more, I have a radio button.
He says, whether he goes to school or not: 

```
<h2>Are you in school?</h2><br>
<input type="Radio" name="ClassRadio" value="No">I'm not any more in school.<br>
<input type="Radio" name="ClassRadio" value="Yes">Yes, in class

```

And here's the problem: After "in class" should come a selection list with 1, 2, 3, etc.  

But if I make this list 

```
<select name="KlasseOption" size="1">
    <option value="1">1</option>
    <option value="2">2</option>
    etc.
</select>

```

You can choose one of the selections in the selection list no matter whether the Radio Button Value "Yes" or Value "No" is pressed.  

But it should only be possible to change the selcetion list if the Radio Button Value "Yes" is pressed.  

If the Radio Button Value "No" is pressed, nobody should be able to change it from the default "--".