Why should people use formal speech at a job interview or at work?
a. informal speech could give the impression that someone is not serious about work.
b. informal speech can be disrespectful in that type of situation.
c. both a and b

C. Both A and B

If you are informal, people tend to think that you don't care that much about what you're doing. In an interview, you can't just go up to the interviewer and say, "Sup, dude" or something else like that. This also plugs into how saying "sup" at the wrong time, in this case a job interview, can also be offending. An interviewer would probably feel awkward and maybe even offended by being addressed so informally.