using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(RemoveDefRows("id,name,age,score\n1,jack,NULL,12\n17,Betty,28,11"));

    }

    public static string RemoveDefRows(string input){

        input = input.Replace("\n", ",");

        string output = string.Empty;

        var arrData = input.Split(',');

        List<string> balance = new List<string>();

        List<string> newBalance = new List<string>();

        string temp = string.Empty;

        int count = 1;

        foreach(var data in arrData){

            temp = string.Concat(temp, ",", data.ToString());

            if(count%4 == 0){

                temp = temp[1..];

                balance.Add(temp);

                temp = string.Empty;

            }

                count++;

        }

        foreach(var data in balance){

            if(!data.Contains("NULL")){

                newBalance.Add(data);

                output = string.Concat(output, "\\n", data.ToString());

            }

        }

        output = output[2..];

        return output;

    }

}