In the problem of processing a line from a keyboard, what are the $3$ options if a backspace is typed when the ADT is empty?

have the program end

have the program restart

have the program ignore the backspace

have the program end with an error message

throw a fit

have the program throw a backspace

have the program end with an error message

have the program sound an alarm

have the program convert the backspace to a letter

have the program end with an error message

throw an exception

have the program ignore the backspace