On a live web server one must turn "DEBUG" off so that any errors on your backend code will not display an error trace on the user's browser. If hackers are able to see error traces and snippets of code they can gain invaluable information about your system that can help them mount attacks against your web application. Which of the following can be used to programatically turn debug on and off, depending on whether your code is running on the local development server or the live web server? Hint: Think about what is normally the same on both development and live servers, and what is normally different.

a. environment variables

b. database configuration c. models

d. view functions

e. static files