Mastery Club Clarification

I am really pleased with all the motivation to master challenges. One thing I have noticed is that students are coming into the classroom and asking if they can do a challenge, but they don't know which one they want to attempt. I told the students they must know which one they want to attempt at the beginning of the day because they have already studied the previous night or few nights. I won't allow a student to take a challenge if they don't know which one that want to attempt until that morning. The idea is they must know it when they first attempt it, and not just know because they learned it in second grade or third grade.