Win

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class Win : MonoBehaviour {
    public Button A;
    public Button B;

    void OnEnable(){
        A.gameObject.SetActive (true);
        B.gameObject.SetActive (false);
    }

    void OnTriggerEnter2D(Collider2D other)
    {
        if(other.tag == "Player")
        {
            B.gameObject.SetActive(true);
        }
    }
}