C# protection level problem

Hello everyone , I'm reading Unity 4.x Game Development by Example Beginner Guide and trying to make sample projects which are in this book. In this book , a lot of code is wrotten in JavaScript. So i want to make a little bit harder this so i'm trying to write codes in C#. In a project a have a problem about protection level problem.

It's giving protection level error in BuildGrid function. I can't reach the class variable from kart.img. It says img is inaccessible due to its protection level.

Can you help me about this ?

So here is my code :

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class GameScript : MonoBehaviour {
 
     // Use this for initialization
 
     public int kolon = 4;
     public int satir = 4;
     public int toplamKart = 16;
     public double kazanmaPuani;
     public int tamamlananMac = 0;
     public int kartW = 100;
     List<Card> kartlar;
     Card[,] aGrid;
     List<Card> donen;
     bool oynanabilir;
     bool kazandi=false; 
 
 
 
     void Start () {
     kazanmaPuani = toplamKart * 0.5;
         oynanabilir = true;
         kartlar = new List<Card>();
         aGrid = new Card[satir, kolon];
         donen = new List<Card>();
         for (int i = 0; i < satir; i++)
         {
             for (int j = 0; j < kolon; j++)
             {
                 aGrid[i, j] = new Card();
             }
         }
 
     }
     
     // Update is called once per frame
     void OnGUI () {
 
         GUILayout.BeginArea(new Rect(0, 0,Screen.width ,Screen.height));
         BuildGrid();
         GUILayout.EndArea();
         print("building grid!");
     }
 
    public class Card
     {
         bool acik = false;
         bool eslesik = false;
         string img;
 
         public Card()
         {
             img = "robot";
         }
 
     }
 
     void BuildGrid()
     {
         GUILayout.BeginVertical();
         for (int i = 0; i < satir; i++)
         {
             GUILayout.BeginHorizontal();
             for (int j = 0; j < kolon; j++)
             {
                 Card kart = new Card();
                 kart = aGrid[i, j];
                 if (GUILayout.Button(Resources.Load(kart.img), GUILayout.Width(kartW)))
                 {
                     Debug.Log(kart.img);
                 }
             }
             GUILayout.EndHorizontal();
         }
         GUILayout.EndVertical();
     }
 }