Which of the following is a directive used to create an "include guard" that allows a program to be conditionally compiled to prevent a header file from accidentally being included more than once? Group of answer choices

#ifndef

Explanation:

#ifndef checks whether the given token has been #defined earlier in the file or in an included file.

It should be understood that #ifndef  preprocessor commands are used to test if a preprocessor variable has been "defined".

And it should be noted that #ifndef is known as header guards. And its primary purpose in C++ is to prevent C++ header files from being included multiple times.

In conclusion , it should be noted that #ifndef is one of the conditional processing blocks, andit is known as header guards because it used in creation of "include guard".

no the answer is c only objects touching the earth's surface can be acted upon by an outside force