Passage Summary: Milk and its sugar help fix bones, and people in the tropics don't drink milk or digest that sugar well, but their bones are just as healthy as everyone else's.

Reasoning: Milk contains lactose, which helps bones; tropical residents lack milk and the ability to process lactose, but their bone health is fine.

Analysis: We have a classic medical mystery: if Factor A (lactose/milk) is a key helper for Process B (bone repair), why are people who lack Factor A doing just fine? To resolve this, we need to find another way these tropical inhabitants are getting their bone-repairing needs met. Perhaps they get calcium from a non-milk source, or maybe they have another way to absorb it that doesn't involve lactose. Look for an answer that provides an alternative explanation for their sturdy skeletons.