True, the included file does not exist it will generate the error.

True, #undef can be used only on a macro that has been #define earlier

Example: #define PI 3.14

We can undefine PI macro by #undef PI

True, We can prevent the same file from getting included again by using a preprocessor directive called #ifndef (short for "if not defined") to determine whether we've already defined a preprocessor symbol called XSTRING_H. If we have already defined this symbol, the compiler will ignore the rest of the file until it sees a #endif (which in this case is at the end of the file).

#ifndef XSTRING_H

#define XSTRING_H defines the same preprocessor symbol,

Finally, the last line of the file, #endif

True, the programmer tells the compiler to include the preprocessor when compiling.

False, A macro just replaces each occurrence with the code assigned to it. e.g. SQUARE(3) with ((3)*(3)) in the program.

A function is compiled once and can be called from anywhere that has visibility to the funciton.