更改'选项'标签

我在使用ajax时遇到了一个小问题。

我需要当我改变'选择'标签的'选项'的值，然后它会调用ajax和我从ajax调用得到的值将显示在'输入'标签名为't_name'。但这行不通。我的代码有什么问题?我怎么解它?谢谢你。

        <select id="teacher_select" class="teacher_select" name="teacher_select" value="Select Teacher">
            <option value="">Select Teacher</option>
            <?php
            while($row=mysql_fetch_assoc($teacher) ):
            ?>
            <option value="<?php echo $row['t_id'];?>"><?php echo $row['t_name'];?></option>
            <?php 
            endwhile;
            ?>
     </select>
         <input type="text" name="t_credit" id="t_credit"/>

     $('#teacher_select').click(function(){
        $.ajax({
        type:'post',
        url:'get_taken_credit.php',
        data: 't_id='+ $('#teacher_select').val(),                
        success: function(reply_data){
            $('#t_credit').html(reply_data);
          }
        });
     });

get_taken_credit.php:

         include('db_connection.php'); 
         $t_id = $_POST['t_id'];
         $result = mysql_query("SELECT t_credit FROM teacher WHERE t_id = '$t_id'");
         echo $result;
         exit();