Which of the following is an external variable in the following C code?

#include <stdio.h>
int func (int a)
{
    int b;
    return b;
}
int main()
{
    int c;
    func (c);
}
int d;

A. Decision making B. Variable declaration C. Code organization and reusability D. Printing output

A. To declare a function's return type B. To declare a function's name C. To declare a function's parameters and return type D. To declare a function's name and parameters

A. 4 B. 8 C. 2 D. Error

A. By value B. By reference C. By pointer D. By address