"They are among us!"
Translation:Elles sont parmi nous !
They are between us means you and another person (or a few) are standing, with "them" in the middle of you. (Or it could be used more emotionally, to mean pulling you and the other person apart, that "they" have come between you, but that would almost always require the "come")
They are among us is a general phrase, such as e.g. "fans of the rival team...they are among us!" as in they exist in the same area we do.
Research a bit on the nuance difference between "among" versus "between" and the 2 vs many distinction. It may help.
I think this is wrong that we should choose both, as parmi and milieu are not the same thing. Parmi means "among" but "au milieu de" means in the middle. When we say in English that we are among friends, we don't mean we are exactly in the middle of friends. So "au milieu de" seems more specific and often more physically located than "parmi".
I like to get an answer from mods or course writers.
That doesn't make any sense. There is no direct object in that sentence. There is never a direct object of the verb "to be".
"Parmi" is a preposition; after a preposition, you use a stressed pronoun - parmi nous, parmi vous, parmi eux, parmi elles.
"Ce" alone, without a noun, doesn't work for people. You can find "ce" in constructions like "c'est" (singular) and "ce sont" (plural), which are equivalent to "there is/are", so these constructions would be followed be nominal groups "ce sont des professionnels", "ce sont des chiens sauvages", "c'est un livre",...