Concept:Mutualism is a relationship where both organisms benefit, while parasitism involves one organism benefiting at the other's expense.
Explanation:In mutualism, both participating organisms receive a positive advantage, such as food, shelter, or protection.
In parasitism, the parasite benefits and the host is harmed or weakened.
Therefore, the main difference is that both organisms gain from the interaction in mutualism.
Answer:A. both organisms involved benefit