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I'm developing a tool that looks at a person's resume and skill set. One of the features is to estimate how much experience a person has based on the length of time on a project and associated skills/tools used in that time (we've had extensive talks about the limitations of this, but that's not really relevant to this question). At the end of the day, I'd like to be able to say "Bob has 4 years experience with C#," but the correct way to show that is unclear to me:
Bob has 4 years experience with C#
Bob has 4 year's experience with C#
Bob has 4 years' experience with C#
I'm writing documentation and the phrase "number of years experience" has shown up a few times, but I think that's the same situation as above (Word grammar checker does not like this phrase).