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));
        print("building grid!");

   public class Card
        bool acik = false;
        bool eslesik = false;
        string img;

        public Card()
            img = "robot";


    void BuildGrid()
        for (int i = 0; i < satir; i++)
            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)))

this line here string img; should be public string img;
just a note any variable in C# without the word public infront of it is considered a private variable in C# where is in java script only variables with private infront of it is considered private .