Edit this code to move object by touch in x & z

I need a script to move object by touch in x and z axis (on the ground) (Y is air). I have found this code from a site, This is suitable code but there is a problem that can move object in X and Y axis, it means you can drag object to the air and i don't want this. I tried to change it myself but i can't ! Please help me to change this code to move object by touch only in x & z axis. (it means by touch x object moves to right and left and by touch y object moves to forward and backward)

 using UnityEngine;
 using System.Collections;
 
 public class Drag : MonoBehaviour {
     public Transform plane;
     private Transform selection = null;
     private Vector3 dist; 
     // Use this for initialization
     void Start () {
 
     }
 
     // Update is called once per frame
     void Update () {
         foreach(Touch touch in Input.touches)
         {
             Ray ray = Camera.main.ScreenPointToRay(touch.position);
             RaycastHit hit;
             RaycastHit point;
 
             switch (touch.phase) 
             {
                 case TouchPhase.Began:
                 if(Physics.Raycast(ray, out hit, 100))
                 {   
                     if (hit.rigidbody !=null)
                     {
                     selection = hit.rigidbody.transform;
                     plane.transform.position = hit.point;
                     }
                 }
                 dist = new Vector3(hit.rigidbody.transform.position.x,hit.rigidbody.transform.position.y,hit.rigidbody.transform.position.z) -  hit.point ;
                 plane.collider.enabled = true;
                     break;
                 case TouchPhase.Moved:
                 int layerMask = 1 << 8;
                 if(Physics.Raycast(ray, out point, 100,layerMask))
                 {
                     selection.rigidbody.constraints = RigidbodyConstraints.FreezePosition;
                     selection.transform.position = point.point + dist;
                 }
                     break;
                 case TouchPhase.Ended:
                 selection.rigidbody.constraints = RigidbodyConstraints.None;
                 selection = null;
                 plane.collider.enabled = false;
                     break;
             }
         }
     }
 }