But I think it is impossible to write such a and b. Can anyone explain this?

It is possible - visibility of the object depends on the scope too, bot only linkage and storage duration.

int a = 1;
static char b = 2;

void y(int a)
{
      //global a is not accessible here
}

//or

void y(void)
{
      int a;
      //global a is not accessible here
}


void x(void)
{
}