A. This receives data from the credential.txt file so it can determine whether a username has been created already and then if it is a new username and password it updates the text file. As for the login function it will go over the credentials file and compare to the user input. B. The login function takes the user input and if the username and password are in the credentials.txt file then the login_succefull.txt file is changed to say run and that will be the signal for the rest of the program to run. C.