Which of the following best states the main point of the passage? A. Before familiar land-based ecosystems could form, both life-forms and the planet’s surface had to change to become better suited for each other.

B. It took a very long time (hundreds of millions of years) for living organisms to colonize dry land after first appearing in the sea.

C. In early geologic times, conditions on dry land were too harsh to support the growth of green plants.

D. A period of intense volcanism in the Ordovician period provided the nutrient-rich soil needed to support green plants on land.