Thanks! I do understand what it means, but MUST you use it? Would omitting "co" make the sentence wrong?
Yes and no.
Technically, I want to maintain the consistency among sentences so usually, without "có", sentence will be marked wrong. Some may have exception.
On the other hand, without "có", the sentence will sound unnatural when used in conversation unless you stress at the end of this sentence (at không) to make it sound like a question.