A、Redwood trees. B、Forest fires. C、San Francisco. D、Survival skills. A主旨题。对话中出现频率最高的词是redwood trees，而且对话中通篇都在讲旧金山沿海的红杉树，讲到其高度、历史

W: (1)Have you ever visited a redwood forest?(2)I recently had a chance to go to Muir Woods National Monument, North of San Francisco.
M: (1)I’ve never seen a redwood tree. 1 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: (2)You said that Muir Woods is near San Francisco? I guess it must be quite a tourist attraction.
W: (2)Yes, it’s less than an hour’s drive away, so it’s easy to get to.
M: I’ve heard that many redwood trees are thousands of years old. Axe the ones in Muir Woods that old?
W: (3)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.(4)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 and see them.
Questions 1 to 4 are based on the conversation you have just heard.
1. What is the main subject of this conversation?
2. Why do many tourists visit Muir Woods rather than other redwood forests?
3. Approximately what is the oldest documented age for a redwood tree?
4. What has contributed most to the redwood survival?