What will be the output of the following C++ code?

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main ()
{
    int first[] = {5, 10, 15, 20, 25};
    int second[] = {50, 40, 30, 20, 10};
    vector<int> v(10);      
    vector<int> :: iterator it;
    sort (first, first + 5);   
    sort (second, second + 5); 
    it = set_union (first, first + 5, second, second + 5, v.begin());
    cout << int(it - v.begin());
    return 0;
}

A. 6

B. 7

C. 8

D. 9

Answer: Option C