Oh long distance relationships can be difficult to reveal to parents, trust me I've been there. Personally you have to tell the truth which such things, whether or not they accept it is a different matter completely. Personally the best way to approach this is talk to your parents and say you met this really great girl, and she wants to meet you. When they start asking questions like how you met and other things be prepared with what you are going to say as it might not be the easiest thing in the world to do however it needs to be done.
The approach, maybe just sit them down, like if you have a dinner all together maybe talk to them during the meal might be one way which might work for them. Telling them separately might be another thing as sometimes it's easier to deal with one parent rather than both at the same time.
Also if they as to talk to her then let them, as they will probably be curious, and whatever questions they throw at you when you are talking about it all, you have to answer them with all honesty because lies tend to catch you up and bit you in the ass.
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