Answer:
Reciprocal altruism is one of the altruism behaviors in which an individual organism helps in increasing the fitness of another organism by reducing the fitness of itself temporarily in expectation of the same behavior from another organism in the future later.
It takes place in the same set of organism partners which is a continuous interaction ion in stable groups of organisms.
Thus, the correct answer would be - an ongoing interaction in the same set of partners.
I would say D. small intestine
If you want an explanation, here it is
After leaving the stomach, water is absorbed mostly in the early segments of the
small intestine, the duodenum, and the jejunum. A small portion of all water absorption occurs in the stomach and the colon: the small intestine absorbs 6.5L/day, whereas the colon absorbs 1.3L/day.
