Mastering C++: A Comprehensive Quiz Based on 'Thinking in C++

Global variables are defined outside of a function and can be accessed by all functions within a file. To access them across different files, the keyword "extern" must be used in the other file. This tells the compiler that the variable is declared in another file and can be accessed. Option A is incorrect because the static keyword restricts the variable's scope to the file it is defined in. Option C is incorrect because redefining the global variable in each file can cause conflicts and may lead to unexpected results. Option D is also incorrect because including the file where the variables are defined will only allow access within that particular file.