表格新增行的奇怪问题
现有两个文件: 
 a.htm 
 ================================================= 
  <html>  
  <head>  
  <title> File   A </title>  
  </head>    
  <body>  
  <input   type= "button "   value= "Add   Row "   onclick= "createRow() "   />  
  <table   id= "tab1 ">  
              <tr>  <td>  <select   onchange= "alert( 'a ') ">  <option> a </option>  <option> b </option>  </select>  </td>  </tr>  
  </table>  
  <iframe   id= "genRow "   style= "display:none "   src= " ">  </iframe>  
  <script   language= "javascript ">  
 function   createRow() 
 { 
             document.getElementById( "genRow ").src= 'b.htm '; 
 } 
 function   addRow(trObj) 
 { 
             var   t   =   document.getElementById( "tab1 ").getElementsByTagName( "tbody ")[0]; 
             //t.appendChild(trObj); 
             t.insertAdjacentElement( "beforeEnd ",trObj); 
 } 
  </script>  
  </body>  
  </html>    
 b.htm 
 ====================================================== 
  <html>  
  <head>  
  <title> File   B </title>  
  </head>    
  <body>  
  <table   id= "tab2 ">  
  <tr>  <td>  <select   onchange= "alert( 'a ') ">  <option> a </option>  <option> b </option>  </select>  </td>  </tr>  
  </table>    
  <script>  
 parent.addRow(document.getElementById( "tab2 ").getElementsByTagName( "tbody ")[0].children[0]); 
  </script>  
  </body>  
  </html>    
 ============================================================= 
 问题描述: 
 当点击a.htm中 "Add   Row "按钮一次的时候新增行的select   onchange事件可用,   但当连续点两次以上时只有最后增加行的select   onchange事件可用,   为什么会这样??
------解决方案--------------------你可以这样 
 b.htm 
 ========== 
  <script>  
 parent.addRow(document.getElementById( "tab2 ").getElementsByTagName( "tbody ")[0].children[0].children[0].innerHTML); 
  </script>    
 a.htm 
 ========= 
  <script language= "javascript ">  
 function createRow() 
 { 
     document.getElementById( "genRow ").src= 'b.htm '; 
 } 
 function addRow(trObj) 
 { 
     var t = document.getElementById( "tab1 ").getElementsByTagName( "tbody ")[0].children[0].children[0]; 
     //t.appendChild(trObj); 
     t.insertAdjacentHTML( "beforeEnd ", " <br>  "+trObj); 
 } 
  </script>
------解决方案--------------------这样试试:   
  <html>  
  <head>  
  <title> File A </title>  
  </head>    
  <body>  
  <input type= "button " value= "Add Row " onclick= "addRow() " />  
  <table id= "tab1 ">  
      <tr>  <td>  <select onchange= "alert( 'a ') ">  <option> a </option>  <option> b </option>  </select>  </td>  </tr>  
  </table>  
  <iframe id= &qu