Which line in the following program contains the header for the showDub function?

```
1 #include
2 using namespace std;
3 void showDub(int);
4 int main()
5 {
6 int x = 2;
7 showDub(x);
8 cout << x << endl;
9 return 0;
10 }
11 void showDub(int num)
12 {
13 cout << (num * 2) << endl;
14 }
```

a. line 3
b. line 4
c. line 7
d. line 11

d. line 11