Choosing Units in Course Challenge

Is it challenging to let teachers assign Course Challenges to students. But I'd love to see a custom-unit Course Challenge where you can pick which units you want to see.

Also, can you please enable infinite loop for all practice content - exercises, quizzes, unit tests, course challenges? Or at least for exercises? This is the missions replacement in Course mastery!

Finally: Can Sal make a tutorial for students on how to use mastery-enabled courses to learn at your own time and pace? This video can cover using Unit Tests and Challenges to accelerate or diagnose gaps, and videos and exercises to drill down on weak areas. There's an amazing Course Mastery tips video already, but I'd love to see a deeper companion to it.