Answer:
B. #
Explanation:
Every preprocessing directive must begin with the # character.
For example:
#define : Used to define a macro
#ifndef : Conditional evaluation of macro
#include : Used to include other preprocessor header file as part of the code
As we can see each of the directives begins with the # character. This is a signal to the preprocessor to interpret the subsequent keyword as a directive.
If I'm not mistaken firewalls should be enough as now the automatically back up all data
