Which Line in the Following Program Contains the Header for the ShowDub

Which line in the following program contains the header for the showDub function?
1 $\quad$ #include <iostream>
2 $\quad$ using namespace std;
3
4 $\quad$ void showDub(int) ;
5
6 $\quad$ int main()
7 $\quad$ {
8 $\quad$$\quad$ int x = 2;
9
10 $\quad$$\quad$ showDub(x) ;
11 $\quad$$\quad$ cout << x << endl;
12 $\quad$$\quad$ return 0;
13 $\quad$ }
14
15 $\quad$ void showDub(int num)
16 $\quad$ {
17 $\quad$$\quad$ cout << (num * 2) << endl;
18 $\quad$ }

A) 4
B) 6
C) 10
D) 15