I am PhD student who was (and still am) trying to find an advisor. Over the summer I contacted many professors, and started small 'trial' projects with them. Since people respond at different times and move at different paces, all of these projects started a few weeks apart, towards the end of the summer. I very recently decided I want to focus on one project with the professor I liked best and want to politely let the other professors know that I no longer want to work with them on their projects. I had not met these other people more than thrice (in three weeks), and had only got to the point of finishing reading a couple of their papers. None of these professors was/is paying me for research, either.
Here's the thing, though - the professors who I don't want to work with are the ones who are willing to take me right away, because I have the skill set they want in their groups. The professor who I want to work with wants to 'test' me for a quarter, so there's always the chance that he might eventually reject me. I however do want to give it a fair shot, since I loved his work, working style and personality the most (we've met twice so far, and both were technical meetings).
So coming to the point, I want to have the other professors that like me as back-ups in case this particular professor is not sufficiently impressed with me by the end of the quarter. How do I convey that to them without pissing them off?