What does the following block of statements do?

Question

What does the following block of statements do?
```c
#ifndef USEFUL_FUNCTIONS
    #define USEFUL_FUNCTIONS
    #include  "useful_functions.h"
#endif
```

Accepted Answer

it prevents the file "useful_functions.h" from being included multiple times

Answer

it includes a library into the source code

Answer

it avoids getting the wrong access path to the file "useful_functions.h"

Answer

it checks that the file "fonctions_utiles.h" does exist

Question from the Language C - Fundamentals test

What does the following block of statements do?