#ifndef checks whether the given token has been #defined earlier in the file or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement. #ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file.
6. You define a structure type globally because _____
Options
A. you save many lines of code by not rewriting an identical structure definition in each function that uses it