6. African lungfish, which are often found in small, stagnant pools of fresh water, produce urea as a nitrogenous waste. What is the advantage of this adaptation? (A) Urea takes less energy to synthesize than ammonia. (B) Small, stagnant pools do not provide enough water to dilute the toxic ammonia. (C) The highly toxic urea makes the pool uninhabitable to potential competitors. (D) Urea makes lungfish tissue hypoosmotic to the pool.