How to move only one object, not multiple objects

Hello! I am implementing moving boxes game. But I have a problem. I want to move only one box in box stack. But, when I lifted some box where is below other boxes, I could lift the box with above boxes. I have to solve this problem! Could you help me? Thank you!

Here is my code for managing mouse.

 using UnityEngine;
 using System.Collections;
 //using ShowMovedText;
 
 public class MouseManager : MonoBehaviour {
 
     //MovedText show_moved;
     public LineRenderer dragLine;
     float dragSpeed = 3f;
 
     Rigidbody2D grabbedObject = null;
 
     //float distance;
 
     void Start() {
 
     } 
 
     void Update() {
         if( Input.GetMouseButtonDown(0) ) {
             // We clicked, but on what?
             Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             Vector2 mousePos2D = new Vector2(mouseWorldPos3D.x, mouseWorldPos3D.y);
 
             Vector2 dir = Vector2.zero;
 
             RaycastHit2D hit = Physics2D.Raycast(mousePos2D, dir);
             //if(hit != null && hit.collider != null) {
             if (hit.collider != null) {
                 // We clicked on SOMETHING that has a collider
                 if(hit.collider.GetComponent<Rigidbody2D>() != null) {
                     grabbedObject = hit.collider.GetComponent<Rigidbody2D>();
                     grabbedObject.gravityScale = 0;
                     dragLine.enabled = true;
 
                 }
             }
         }
 
         if( Input.GetMouseButtonUp(0) && grabbedObject != null ) {
             grabbedObject.gravityScale = 1;
             grabbedObject = null;
             dragLine.enabled = false;
         }
     }
 
     void FixedUpdate () {
         if(grabbedObject != null) {
             // Move the object with the mouse
             Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             Vector2 mousePos2D = new Vector2(mouseWorldPos3D.x, mouseWorldPos3D.y);
 
             Vector2 dir = mousePos2D - grabbedObject.position;
 
             dir *= dragSpeed;
 
             grabbedObject.velocity = dir;
         }
     }
 
     void LateUpdate() {
         if(grabbedObject != null) {
             Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             Vector2 mousePos2D = new Vector2(mouseWorldPos3D.x, mouseWorldPos3D.y);
             dragLine.SetPosition(0, new Vector3(grabbedObject.position.x, grabbedObject.position.y, -1));
             dragLine.SetPosition(1, new Vector3(mousePos2D.x, mousePos2D.y, -1));
         }
     }
 
 }