ATP, Heat, and Carbon dioxide
I would say the answer is b.
Answer:
If you didn't have an excretory system, or you had one that seldom worked/worked poorly, you would have a very poor time. Firstly, you might be subject to the nasty experience of vomiting your own fecal matter, because your body needs to get rid of it somehow. You would also experience lots of stomach/gastrointestinal pains.
(I'm sorry I took so long!! Hoping it's still useful)
The readings will generally be quite low. :)