Here are the best choices for each blank based on the context of Buck's journey and experiences in "The Call of the Wild":
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(B) sat outside the camp and howled for the return of the wolf, refusing to leave Thornton’s side even for an hour.
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(A) might well have caused him to be mistaken for a gigantic wolf, larger than the largest of the breed.
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(B) patiently and repeatedly attacking it over a period of four days.
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(A) Thornton, his partners, and the other dogs had been attacked and killed by Yeehats.
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(B) ripped the throats of the Yeehats, causing them to flee in terror from what they proclaimed an Evil Spirit.
These choices reflect Buck's transformation and the relationships he forms throughout the story.