I am currently involved in two very different Biology research labs in two different areas at my university, as an undergraduate volunteer. Lab A has been a long-term commitment of mine, and Lab B is a newer addition to my schedule but also a valuable experience, I think, but I have a choice coming up:
Before I graduate, I must complete one (and only one) senior capstone/thesis/etc lab project, that will almost certainly take up all the time I can dedicate to research outside of my regular coursework. I originally sought out and joined Lab B because it was looking like Lab A would not be able or willing to advise a capstone project, but now that is no longer the case - so I want to complete all my senior work at Lab A.
How do I politely tell Lab B that I do not plan to complete a capstone there after all, and probably won't even remain a volunteer in future semesters? (I don't recieve any payment or grades from them, but I know it does take time and effort to train new lab members so I feel bad about leaving without contributing a paper or presentation or something.)