500
Tara is right because of the process of succession. A lake begins with no living things, and then soil is carried into the lake. The soils adds organisms that help the lake support small plants. Animals come to eat the plants, and the lake gradually fills with soil. It becomes a marsh, and when the marsh dries up, a forest grows.
Harry and Tara are hiking in a forest. Tara tells Harry that the forest was probably a lake at one time. Harry doesn't believe it. Explain why Tara is right.