Please solve this. This is my nptel question?

// C# program to compute submatrix
// query sum in O(1) time
using System;
 
class GFG
{
    static int M = 4;
    static int N = 5;
     
    // Function to preprocess input mat[M][N].
    // This function mainly fills aux[M][N]
    // such that aux[i][j] stores sum of
    // elements from (0,0) to (i,j)
    static int preProcess(int [,]mat, int [,]aux)
    {
        // Copy first row of mat[][] to aux[][]
        for (int i = 0; i < N; i++)
            aux[0,i] = mat[0,i];
         
        // Do column wise sum
        for (int i = 1; i < M; i++)
            for (int j = 0; j < N; j++)
                aux[i,j] = mat[i,j] + aux[i-1,j];
         
        // Do row wise sum
        for (int i = 0; i < M; i++)
            for (int j = 1; j < N; j++)
                aux[i,j] += aux[i,j-1];
                 
        return 0;
    }
     
    // A O(1) time function to compute sum
    // of submatrix between (tli, tlj) and
    // (rbi, rbj) using aux[][] which is
    // built by the preprocess function
    static int sumQuery(int [,]aux, int tli,
                        int tlj, int rbi, int rbj)
    {
        // result is now sum of elements
        // between (0, 0) and (rbi, rbj)
        int res = aux[rbi,rbj];
     
        // Remove elements between (0, 0)
        // and (tli-1, rbj)
        if (tli > 0)
            res = res - aux[tli-1,rbj];
     
        // Remove elements between (0, 0)
        // and (rbi, tlj-1)
        if (tlj > 0)
            res = res - aux[rbi,tlj-1];
     
        // Add aux[tli-1][tlj-1] as elements
        // between (0, 0) and (tli-1, tlj-1)
        // are subtracted twice
        if (tli > 0 && tlj > 0)
            res = res + aux[tli-1,tlj-1];
     
        return res;
    }
     
    // Driver code
    public static void Main ()
    {
        int [,]mat = {{1, 2, 3, 4, 6},
                      {5, 3, 8, 1, 2},
                      {4, 6, 7, 5, 5},
                      {2, 4, 8, 9, 4}};
                         
        int [,]aux = new int[M,N];
         
        preProcess(mat, aux);
         
        int tli = 2, tlj = 2, rbi = 3, rbj = 4;
         
        Console.Write("\nQuery1: " +
                      sumQuery(aux, tli, tlj, rbi, rbj));
         
        tli = 0; tlj = 0; rbi = 1; rbj = 1;
         
        Console.Write("\nQuery2: " +
                      sumQuery(aux, tli, tlj, rbi, rbj));
         
        tli = 1; tlj = 2; rbi = 3; rbj = 3;
         
        Console.Write("\nQuery3: " +
                      sumQuery(aux, tli, tlj, rbi, rbj));
    }
}