OnMouseDrag, how to make the object not move on the Y axis

Hello, i found a problem and don't know what to do. i am able to move my cube, but it goes in the Y axis and X and Z. i only want the cube to move in the X Z axis. how do i stop the cube going up?

using UnityEngine; using System.Collections;

[RequireComponent(typeof(BoxCollider))] [RequireComponent(typeof(MeshRenderer))]

public class DragRigidBody : MonoBehaviour { Vector3 screenPoint, offset, scanPos, curPosition, curScreenPoint;

 public float gridSize = 0.20f;

 void OnMouseDown()
 {
     scanPos = gameObject.transform.position;
     screenPoint = Camera.main.WorldToScreenPoint(scanPos);
     offset = scanPos - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));

 }




 void OnMouseDrag()
 {
     curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
     curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;


     curPosition.x = (float)(Mathf.Round(curPosition.x) * gridSize);
     curPosition.z = (float)(Mathf.Round(curPosition.z) * gridSize);

     transform.position = curPosition;
 }

}