Extern variables question

[John Hascall]

    I have been involved in a ``discussion'' over which (if any) of the
four following pairs of modules are identical in result.  Assume that in
each case the two modules are compiled separately and linked with:

     int main(int argc, char **argv) {
        void foo(void); void bar(void); foo(); bar(); return(1); }


          module1.c             module2.c
          -------------------   -------------------
   (a)    int i;                int i;
          void foo(void) {}     void bar(void) {}

   (b)    extern int i;         int i;
          void foo(void) {}     void bar(void) {}

   (c)    extern int i;         int i = 0;
          void foo(void) {}     void bar(void) {}

   (d)    extern int i;         extern int i;
          void foo(void) {}     void bar(void) {}

What is the answer and WHY?

Thanks, John