The answer is D. <em>While </em>and <em>however</em>.
A doesn't work because "both" is a word of comparison, B isn't right because "also" is a transitional word and has nothing to do with contrasting the two animals, and C is incorrect because "similar" isn't a word of contrast; instead, it's a word you use when you compare two things.
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The first act of revenge that Hrothgar mentions is Grendel's mother, "That errant evil", capturing his advisor <span>Aeschere and feasting on him. She does this to avenge the death of her son, Grendel. Another act of revenge is what Beowulf must do, to kill Grendel's mother to avenge Aeschere's death. In Hrothgar's view (and according to the Anglo-Saxon idea of justice), Beowulf must exterminate this daemon to restore the king's order on Earth.</span>
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