Opened. A participle is a word formed by a verb. Example: Going, Gone, being, been. Etc
My best bet would to go with 'were' because they sound similar and it just seems more professional.
Third person plural pronoun is THEY.
If you want to create a possessive pronoun out of that one, that would be THEIRS.
Possessive pronouns, as the name itself says, determine some kind of possession. For example:
Is this books yours? - No, it's theirs.
Answer:
Its conflict. Im pretty sure. If not, try resolution.
Explanation:
Logical thinking.
