Prove for each:
a. Proposition: If A is finite and B is countable, then A ∪ B is countable.
b. Proposition: Every subset A ⊆ N is finite or countable. [Similarly if A ⊆ B with B countable.]
c. Proposition: If N → A is a surjection, then A is finite or countable. [Or if countable B → A surjection.]
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