title: Dropdown lists - two options
body: <p>i have two dropdownlists:
  user can choose values and when he cliked on button it redirect him on the wwww...</p>
  
  <pre><code>function loadNew()            {          
  selectLetter = document.forms[0].letter.value.toString() + document.forms[0].number.value.toString();               
  switch (selectLetter)                {                    
  case  "12":             
  location.href = "http://";                        
  break;                  
  case "21":                      
  location.href = "http://";
  break;                  
  
  default:                      
  location.href = "http://";                        
  break;                }   
   }                                
   &lt;/script&gt;    
   &lt;/head&gt;
  </code></pre>
  
  <p>Body:</p>
  
  <pre><code> &lt;body onLoad="visibilityT()"&gt;        &lt;form onSubmit&gt;        &lt;select name="letter" id="letter" "&gt;
   &lt;option&gt;&lt;/option&gt;
   &lt;option&gt;1&lt;/option&gt;
   &lt;option&gt;2&lt;/option&gt;
  
   &lt;/select&gt;&lt;select name="number" id="number"&gt;
   &lt;option&gt;&lt;/option&gt;
   &lt;option&gt;1&lt;/option&gt;
   &lt;option&gt;2&lt;/option&gt;
  
   &lt;/select&gt;             
   &lt;input type="button" value="Submit" onClick="loadNew()"&gt;        
   &lt;/form&gt;    
  </code></pre>
  
  <p>How to disable option that user can choose the same values?
       </p>