Answer:
the
Explanation:
wouldn't be "a" because "an" is used before a word starting with a vowel
dictionary states "the" is "used to refer to a person, place, or thing that is unique"
I'd like to think it adds a tone of disgust. "Horrible vermin", as in "gross creature."
Trojan (one of the most complicated threats)
It is a phrase that is commonly said and it doesn’t compare any two things, so it is an idiom.