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In which kind of business are you? I have heard this sentence lately but I am not totally sure it is correct. What do you think?
Comments  
I prefer 'what line of business are you in?'.
Totally agree. It sounds a lot more refined.
Teachers: We supply a list of EFL job vacancies
"What business are you in?" would do the job as well.

Google's found only 233 occurrences for "what line of business are you in" against 3,840 for "what business are you in".