How to make a comparison between singlethread and multithread?

I want to make a comparative analysis of the processing time of the code below in a singlethread and in a multithread, but I don't know where I should start the threads, and how to implement multithread. Should I split the code to make multithreads?

I already implemented the function clock_t to calculate the time that it is going to take, and the code is working. I just need to add threads to it, and to make a comparison to see which one is faster: singlethread or multithread.

BONUS: Can you make it faster using OpenMP?

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
// R = Row
// C = Column
void fill4x4( int a[][4] ) {
   for( int r = 0; r < 4; r++ )
       for( int c = 0; c < 4; c++ )
           a[r][c] = rand() % 100 + 1;
}

void show4x4( int a[][4] ) {
   for( int r = 0; r < 4; r++ )
       for( int c = 0; c < 4; c++ )
           printf( "%5d%c", a[r][c], "   \n"[c] );
    puts( "" );
}

int showPair( int a, int b, int pos ) {
   printf( "%2dx%-2d%c", a, b, "   \n"[pos] );
   return a * b;
}

void mult4x4( int a[][4], int b[][4], int d[][4], bool x ) {
   for( int r = 0; r < 4; r++ )
       for( int c = 0; c < 4; c++ )
           // assign (=), not accumulate (+=)
           // notice the 'exchange' of row/col access of 'b[][]'
           if( x )
               d[r][c] = showPair( a[r][c], b[r][c], c );
           else
               d[r][c] = showPair( a[r][c], b[c][r], c );
   puts( "" ); show4x4( d );
}

int main()
{
   srand(time(0));
   double time_spent = 0.0;

   clock_t begin = clock();

   int a[4][4]; fill4x4( a ); show4x4( a );
   int b[4][4]; fill4x4( b ); show4x4( b );
   int d[4][4];
   mult4x4( a, b, d, true );
   mult4x4( a, b, d, false );

   clock_t end = clock();
   time_spent += (double)(end - begin) / CLOCKS_PER_SEC;
   printf("Foram gastos %f segundos.", time_spent);
   return 0;
}