Passage Summary: To be a good zoo, it must be well-managed. All big, well-managed zoos have full-time vets on staff, and many of these big, good zoos stay open all year.

Reasoning: Valuable zoos must be well run; all well-run large zoos have full-time veterinarians; many large valuable zoos are open year-round.

Analysis: We can chain these facts together like a logic puzzle. We know that 'Valuable' implies 'Well Run.' We also know that 'Large' + 'Well Run' implies 'Full-time Veterinarians.' Therefore, if you have a zoo that is both 'Large' and 'Valuable,' it must be 'Well Run,' which means it must have full-time veterinarians. Look for an answer that reflects this necessary deduction. Don't be distracted by the 'most' or 'many' statements regarding small zoos or winter hours, as they don't provide the same level of certainty.