ANS:
The part in front of the apostrophe is always the subject, so you can easily figure it out.
So for singular possessive, take the singular,”country”, and add “ ’s ” “Belonging to one country”
For the plural possessive, take the plural, “countries” and add “ ‘s”. “Belonging to many countries.”
When that results in “s’s”, (countries’s) , the second s is dropped, hence, “countries’ “