Differences between global variables and global static variables

Differences between global variables and global static variables:
(A) if the program is composed of a source file, there is no difference between the global variable and the Global Static variable.
(B) if the program is composed of multiple source files, the global variable is different from the Global Static variable. The Global Static variable makes the variable exclusive to the source file defining the variable, that is: the global static variables are invalid for other source files that make up the program.
(C) static files with external links; can be called in all source files; other files can be referenced in extern mode except this file;
Static global variables:
(A) do not worry that other source files use the same variable names and are independent of each other.
(B) static global variables defined in a source file cannot be used or modified by other source files.
(C) it can only be used in this file! Static with internal links; cannot be called in other files;

Differences between global variables and global static variables