Answer:
B. Start slowly and gradually build up intensity
I would go with A) Yoga, its a safe way to move and not B) because that's very difficult, not D) because u can get hurt, and not C) because u can pop a joint.
B) 12 ft. to 25 ft.
Since they are in different grades they are likely in different classes, the only time their day would overlap might be in cafeteria or lunch time, and even then they wouldn't sit close to eachother.