The Y works perfectly well, but the X (left and/or right) just makes the sprit travel on infinitely.

Question: What's wrong with it? and how do I fix it?

 #pragma strict
 
 var moveUp : KeyCode;
 var moveDown : KeyCode;
 var moveRight : KeyCode;
 var moveLeft : KeyCode;
 
 var speed : float = 5;
 
 function Update () {
         if (Input.GetKeyDown(moveLeft)){
             rigidbody2D.velocity.x = speed;
             
         }    else if (Input.GetKeyDown(moveRight)) {
             rigidbody2D.velocity.x = speed * -1;
         
         }    else if (Input.GetKeyDown(moveUp)) {
             rigidbody2D.velocity.y = speed;
         
         }    else if (Input.GetKeyDown(moveDown)) {
             rigidbody2D.velocity.y = speed * -1;
         
         } else {
             rigidbody2D.velocity.y = 0;
             rigidbody2D.velocity.x = 0;
         }
 
     }