Concept:Pigs are valued in livestock farming mainly for their very high reproductive ability.Explanation:Pigs give birth to many piglets per litter and can farrow more than once each year.They are monogastric animals, so they cannot digest cellulose well.They are not primarily kept as milk producers and are not notably resistant to parasites.Thus, the key characteristic of pigs is that they are highly prolific.Answer:A. highly prolific