All the answers seem to be right, how do I know which one to pick?

Hey Patrick,

It helps if we diagram these problems, as the logic can be hard to see otherwise.

Here are the diagrams:

Hatfield → can't farm.
McCoy → can farm.
McCoy → not Hatfield
not a Hatfield → horseback rider

Now, let's look at the answer choices:

(A) The only people who can farm are horseback-riding McCoys.
- This won't work. We only know that the Hatfields can't farm, and that the McCoys can. We can't conclude anything about ALL other people, which we'd have to be able to do in order to draw the conclusion in this answer choice.

(B) Anyone who does not belong to the McCoy clan belongs to the Hatfield clan.
- Again, we can't say this is true. What about the Jones family? The Smith family? (The problem here is the "anyone.")

(C) All horseback riders can farm.
- This doesn't make any sense. We know that if you're not a Hatfield, you're a McCoy. And we know that if you're not a Hatfield, you must be a horseback rider. We can't say anything about ALL horseback riders.

(D) All horseback riders must be McCoys.
Again, we can't conclude anything about ALL horseback riders. None of the diagrams above allow us to.

(E) All McCoys are horseback riders.
* This follows directly from our diagram: McCoy → not a Hatfield → horseback rider

Choice (E) is our winner. See why diagramming is useful?