"Must be true" statements usually involve assumptions, something that an argument can't go without . Assumptions are the unstated premises of an argument. Think about the following argument:
We have our Spring Dance next week. I don't think that Alice will have a date for it, though, because she's gained a lot of weight recently.
Here, I am making the assumption that the guys in our circle of friends don't like girls with a slight weight issue. This assumption is necessary for the argument to hold. If I were to negate this assumption, then the argument falls apart. If boys do like heavier girls, then Alice stands a good chance of getting invited to the dance.
Now, "could be true" statements are usually deduced from an argument. They're not 100% bound to be true, though, because you might need extra info to be completely sure.
A could be true because, if the southern part was a vast ocean, it is reasonable to assume that the northern part was less so.
B could also be true. Even though the southern half was covered by water and plesiosaurs live in water, there's no guarantee that you'll find them there. They might like only fresh water, while the water covering the southern part of the region could have been salty.
C can't be true. We're told that plesiosaurs lived during the Cretacious period exclusively, so there's no way to find them there before this period. C is the exception.
D is indeed a trap, but you need to remember that we are discussing a "certain region" only here and not the entire Earth. Maybe dinosaurs just did not live in that part of the planet, who knows? Even if it's reasonable to assume that the northern part was free from water and thus inhabitable by dinosaurs, you don't know for sure if they were actually there.
E is a bit odd, so to speak... It also seems that it can't be true! Maybe someone can help me out here. So dinos did not go into water, which means that they did not live in the southern part of the region for the Cretacious period. This means that they did not live there for the entire Mesosoic era... But if I had to choose, I'd go for C!
