พี่ครับ อยากทราบว่า ใช้ ถ้าใช้ dropdown แบบ ว่า ถ้าผมเลือกำ นศแล้ว นศหายไปควรทำยังไงอะครับ รบกวนช่วยที่ครับ

<?php

$con=mysqli_connect("localhost","root","","ae2de");

// Check connection
if (mysqli_connect_errno($con))
{
  echo "Failed to connect to MySQL: " . 	mysqli_connect_error();
}

mysql_query("SET NAMES UTF8");

function utf8_urldecode($str) {
        return html_entity_decode(preg_replace("/%u([0-9a-f]{3,4})/i", "&#x\\1;", urldecode($str)), null, '');
	
}



$teacher_id=$_POST["TC"];
$student_id=$_POST["Student"];
$work_id=$_POST["week"];






$result = mysqli_query($con,"INSERT INTO assing  VALUES ('','$teacher_id', '$student_id', '$work_id',NULL ,'' ,'',NULL,'')");

echo "Sign up Complete..**  ". $student_id;

	mysqli_close($con);
	
	?>

 function work_st(assing_id)
        
            
        {  
            var str =	
                        "TC="+(document.getElementById("Tc").value)+"&"+
                        "Student="+(document.getElementById("Student").value)+"&"+
                        "week="+document.getElementById("week").value;
            
           
            
                intel.xdk.device.getRemoteData(
            		    "http://localhost/Ae2DE/Work_ass.php",
            		    "POST",
            		     str,
                        "success_gelAllwork_ass",
            		    "error_handler1");
          
           
        }
      
          function success_gelAllwork_ass(data)
        {
           var str = "";
                alert( str + " " + data);
        }