I believe its A. Demonstrate responsibility making the restrictions unnecessary .
Mark Twain wants to add religious overtones (b)
Because St. Bernard is a dog but has a slightly religious name and a Presbyterian is a dog but also a British church.
Hope it helped,
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To be really honest, I think its a matter of opinion on which sentence is the strongest conclusion. To me, I think choice C is the best answer because it paints the contrast between the dog's wild instincts versus loyalty. I guess the conclusion should also mention that despite the abuse, Buck still learned to trust a human (even though one could easily understand why the dog would never trust a human after the abuse), so it shows just how powerful loyalty can be.