Final answer:
The correct answer is 'c. to have flourished,' indicating that dawn redwoods flourished around 100 million years ago. These trees have experienced changes in distribution due to environmental factors, and modern redwoods form part of a climax community subject to ecological changes.
Step-by-step explanation:
The correct answer to the question "the dawn redwood appears ... some 100 million years ago in northern forest around the world." is c. to have flourished. This answer choice uses the correct infinitive form to express that the dawn redwood flourished in the past, which is evident from the paleobotanical record and the discovery of related fossils.
Dawn redwoods are part of a long history of plant evolution. During the Mesozoic era, they were widespread across many parts of the world. However, the presence of these trees has varied over time due to climate changes, continental drift, and other environmental factors. Over the course of millions of years, their distribution has changed. Initial forests emerged over 300 million years ago, evolving and supporting a variety of plant species, including ancestors to the modern dawn redwood.
Today, redwood trees are restricted to specific regions and are often associated with a climax community, indicating the final stage of ecological succession. These ecosystems are sensitive to changes such as logging, pollution, and climate changes, such as global warming, which affects essential fog patterns they rely on for water.