3. Of the three ways to pass arguments to functions, only passing by _____ and passing by _____ allow the function to modify the argument in the calling program
#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.
10. To be called object-oriented, a programming language must allow