Consider the following five functions:

char* f1(char* s)
{
  s += 5;
  *s = 0;
  return s; 
}
char* f2(char* s1)
{
  char* s2 = s1;
  s2[5] = '\0';
  return s1; 
}
char* f3(char* s1)
{
  char s2[5] = s1;
  return s2;
}
char* f4(char* s)
{
  return strstr(s, "Hello");
}
char* f5(char* s1)
{
  char* s2 = NULL;
  strncpy(s2, s1, 5);
  return s2;
}

and consider the following variable:

char string[] = "Hello world!";

For each of the functions, choose the appropriate statement when called on string.