阅读下列短文，从每题所给的A、B、C和D四个选项中，选出最佳选项。
A
I have a pretty good idea of how big Lake Superior is because I walked all the way around it. I walked about ten miles a day, and it took me five and a half months.
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1.
Lake Superior was created by a glacier a mile thick, which pushed south from Canada. It dug a huge basin that filled with water when the glacier melted about ten thousand years ago. All the soil was blown off, leaving behind only hard rocks.
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2.
The Cascade River falls over rocky pools before running into the lake. (Cascade River State Park, Minnesota)
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3.
This is one of my favorite places on the trip. Here the trail runs over some of the oldest and hardest rocks on Earth. (Pukaskwa National Park, Ontario, Canada)
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4.
This rock painting was made by Ojibway Indians. It records a crossing of the lake by about fifty men in four or five canoes. (Lake Superior Provincial Park, Ontario, Canada)
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I was surprised to be treated in such a special way by the people I met on my trip. I got gifts of food and invitations to share meals and stay in people's homes. Somehow the trip was important to these people, too. People asked me questions like “What kind of food do you eat？” and “Don't your feet get tired？” I'm glad nobody asked me why I did the walk. I'm not exactly sure myself. All I know is that it was the best thing I've done in my life.
21．Which picture and caption shows the origin of Lake Superior?
A.The first. B．The second. C．The third. D．The fourth.
22．What's the geological feature of Lake Superior area?
A.Hilly. B．Sandy. C．Rocky. D．Muddy.
23．What does the writer think of the trip?
A.Tiring. B．Surprising. C．Adventurous. D．Wonderful.
