A、Redwood trees. B、Forest fires. C、San Francisco. D、Survival skills. A主旨判断题 对话中出现频率最高的词是redwood trees，因此可以判断红杉树是本文的主要话题。

W: Have you ever visited a redwood forest? I recently had a chance to go to Muir Woods National Monument, north of San Francisco.
M: I’ve never seen a redwood tree. I really can’t imagine how big they are.
W: The coastal redwoods are the tallest living things. Some are more than 350 feet high. But none of the trees in Muir Woods is that tall. You have to go further north in California to see the tallest trees.
M: You said that Muir Woods is near San Francisco? I guess it must be quite a tourist attraction.
W: Yes, it’s less than an hour’s drive away, so it’s easy to get there.
M: I’ve heard that many redwood trees are thousands of years old. Are the ones in Muir Woods that old?
W: The oldest documented age for a coastal redwood is more than 2,000 years. The trees in Muir Woods are 400 to 800 years old.
M: Why have they survived so long?
W: They have remarkable resistance to forest fires. Their tough, thick bark protects the trees during a fire. The coastal redwoods also like a damp, foggy climate.
M: Then, since Muir Woods is near foggy San Francisco, it must be ideal for the tree’s survival. I can’t wait to go there.