Which of the following would make the best topic sentence for a paragraph about freshwater swamps?

• A. Freshwater swamps play a very important role in not only balancing the natural ecosystem, but also protecting human development from the elements.
• B. In times of heavy rain, swamps can help control flooding by acting as basins to collect large amounts of precipitation.
• C. Swamps are home to countless varieties of wildlife, including fish, frogs, insects, and reptiles.
• D. Many of the plants in freshwater swamps act as "nature's kidneys," purifying the water and filtering out harmful pollution.