Question

click to correct any capitalization errors.
after he retires, my grandfather plans to visit the base of mount everest and take a boat ride down the amazon river.
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Explanation:

Proper nouns (specific geographic landmarks like mountain names) need capitalization of all key words. Common noun phrases like "boat ride" should only capitalize the first word if starting a sentence, not in the middle of a sentence.

Answer:

After he retires, my grandfather plans to visit the base of Mount Everest and take a boat ride down the Amazon River.