Initially we were only going to give the 9 materials that were actually used as options, then we thought it may be too easy. On reflection you are probably right in that it is hard. We have not run a competition with this format before so it will be a learning experience for us as well. in the end it will be the person with the most correct answers that will win so luck is probably going to have a lot to do with it, best guess basically.

If you click on the image it expands to 1000x1000 pixels if that is of any help.