Code to multiply two matrices using for loop and 2d arrays -

Below is the code for such program -

#include <iostream>
using namespace std;

int main() {
    
    int r1, c1, r2, c2;
    
    // Input dimensions
    cin >> r1 >> c1;
    cin >> r2 >> c2;
    
    // Check multiplication condition
    if (c1 != r2) {
        cout << "Matrix multiplication not possible";
        return 0;
    }
    
    int A[100][100], B[100][100], C[100][100];
    
    // Input first matrix
    for(int i = 0; i < r1; i++) {
        for(int j = 0; j < c1; j++) {
            cin >> A[i][j];
        }
    }
    
    // Input second matrix
    for(int i = 0; i < r2; i++) {
        for(int j = 0; j < c2; j++) {
            cin >> B[i][j];
        }
    }
    
    // Initialize result matrix
    for(int i = 0; i < r1; i++) {
        for(int j = 0; j < c2; j++) {
            C[i][j] = 0;
        }
    }
    
    // Matrix multiplication
    for(int i = 0; i < r1; i++) {
        for(int j = 0; j < c2; j++) {
            for(int k = 0; k < c1; k++) {
                C[i][j] += A[i][k] * B[k][j];
            }
        }
    }
    
    // Output result matrix
    for(int i = 0; i < r1; i++) {
        for(int j = 0; j < c2; j++) {
            cout << C[i][j] << " ";
        }
        cout << endl;
    }

    return 0;
}

----
But I have some questions regarding some specific parts in the code. That is,
1) Why is there a third nested loop?

(int k = 0; k < c1; k++)

2) Can we write variable 'r1/r2' , 'c1/c2' instead of the integer "100" in array index below? -

 A[100][100], B[100][100], C[100][100];