A comma because a semicolon is used to separate 2 different sentences.
The theme of reward can be seen from the line "The first thing which I saw filled me with amazement".
- I was amazed by the first thing I noticed. In every direction, I could make out ruddy, distinct patches of light that resembled ship's portholes in the night. I briefly believed that it was the lava-glow from some volcanic activity, but this was improbable. Any volcanic activity would undoubtedly take place below the hollow, not above the rocks. What was the other option, then? Even though it was lovely, it must be. The reflection of fires inside the caves must be the cause of these ruddy blotches. At that time, there were people living on the plateau. How admirably well-founded my expedition was! This was definitely news that we could bring back to London!
- In his 1912 science fiction book The Lost World, British author Sir Arthur Conan Doyle describes an expedition to a plateau in South America's Amazon basin where extinct species still roam free.
Thus this is the theme of reward.
To learn more about Arthur Conan Doyle, refer: brainly.com/question/20595453
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The answer is c. a metaphor