Cast a ray to select 2D sprite in Unity

At beginning, I hope my grandfather could get better and move out of ICU  as soon as possible.

In Unity, we usually cast a ray from main camera to select a sprite. The sprite need to be added a collider2D component to realize the function.

void Update () {
  ClickItem ();
}

void ClickItem(){

  //check if the left button of mouse has been clicked

  if (Input.GetMouseButtonDown (0))

  {

    //transfer the position values to that in world axis    Vector3 v3 = Camera.main.ScreenToWorldPoint(Input.mousePosition);
    Vector2 v2 = new Vector2 (v3.x, v3.y);

    //cast a ray from zero point which is also the position of camera to v2    RaycastHit2D hit = Physics2D.Raycast (v2, Vector2.zero);

    if (hit.collider != null) //if the ray hit something

    {
        //Debug.Log(hit.collider.tag);

        //if the collision happens on an object itself
        if (hit.collider.gameObject == this.gameObject) { 

            ..............

        }

    }

  }

}