When the immune system attacks itself.
Stretching exercises would maintain his agility and flexibility without causing heavy force on his bones.
I would think that the answer is D. all of these, considering that:
1) The cytoplasm is mostly made of water, but also contains salt, enzymes, organelles, and orgaince molecules; 2) The cytoplasm supports and suspends cellular molecules and organelles; and 3) Chemical reactions also occur in the cytoplasm, and the enzymes it contains help speed up these reactions.
