At the moment, the green bubble is filling in no matter what a student picks. This is currently an engineering issue and it's related to the fact that students get the success sound and the puzzle solves, no matter what was selected. Our engineers are working on a fix to give a sound that corresponds to the success or failure of the answer decision, rather than the puzzle solution, and this should fix the bubble issue as well. At which time, the "I don't know." answer will be marked as not correct.
Keeping the "I don't know" option allows us to see whether students are trying to figure these out, or just trying to get through. It also helps us see whether a student believes that they are picking something that will work, or if they've given up. Our hope is that this data will help us make the stages better.
Thanks for your feedback!