> **3.** We saw that writing `#include <file>` instead of `#include "file"` may not work if file is one that we've written. Would there be any problem with writing `$include "file"` instead of `#include <file>` if *file* is a system header?
Yes, `""` is a path relative to the current file, whereas `<>` is a path to the system's standard library headers.
---
> **4.** Assume that `debug.h` is a header file with the following contents...
>(a). What is the output when the program is executed?
> (c). Explain why the output is different in parts (a) and (b)
When `DEBUG` is defined, any instances of the `PRINT_DEBUG` token are replaced with a `printf` call during compile time, but when it's not defined, they're replaced with nothing.