Premises —\n1) “None but the rich can afford air travel” (i.e., only rich people can afford air travel).\n2) Some of those who travel by air become sick.\n3) Some of those who become sick require treatment.\nQuestion — Which conclusion is valid?

Introduction / Context:
We are given a classic “only” statement (“none but the rich…”) and two additional existential premises about sickness and treatment among air travelers. The task is to identify which conclusion is logically entailed, not merely plausible.

Given Data / Assumptions:

• “None but the rich can afford air travel” means: If a person can afford air travel (or travels by air), then that person is rich. Symbolically, AirTravel → Rich.
• “Some who travel by air become sick” and “some of those sick require treatment” add detail but do not alter the “only rich afford” mapping.
• No claim is made that all rich travel by air, or that all air travelers become sick.

Concept / Approach:
Translate “only” carefully. The correct inference from “only rich can afford air travel” is that every air traveler belongs to the set of rich people. It does not say the reverse (that every rich person travels by air). Evaluate options accordingly.

Step-by-Step Solution:

Verification / Alternative check:
Try a model: Let 10 rich people exist, 3 travel by air, and 2 of those get sick, 1 needs treatment. All statements hold and D remains true. No contradiction occurs.

Why Other Options Are Wrong:

Common Pitfalls:
Misreading “only” statements. Remember: “Only rich travel by air” means “air traveler → rich,” not “rich → air traveler.”

Final Answer:
All those who travel by air are rich.