The Carotid-Sinus Receptors are part of a complex chain of nerves regulating arterial pressure and if the range they regulate exceeds a predetermined limit they signal for a reduction of that pressure by slowing the heart down.
The best thing to do with these kinds of questions is to establish a baseline for any additional calculations by taking the object up to the surface. This makes the surface the constant to use for the rest of the question.
If it's taken up through 25m/sw it will expand 3.5 times (25m/sw=3.5ATA). This will make the object 70% of its total volume (3.5x20=70).
We then take the object down to 10m/sw which is 2 ATA (70/2=35)
The revised Advanced Open Water Course includes Adventure Dive options from Rebreather, Ice, Self-Reliant and Cavern Specialty Courses which have a minimum prerequisite age of 18 years old.
This can be checked in the Instructor Manual - Advanced Open Water and there is a pretty easy to reference chart showing all pertinent details about Adventure Dives available for the program.
The recycling of the breathing gas while scrubbing carbon dioxide and supplementing oxygen into the system is undoubtedly the present and future of diving. Read more about these units in your PADI Encyclopedia of Recreational Diving.
CCR is on my 'to-do' list for sure.