Passage Summary: Scientists found old diamonds with a specific chemical that only forms when hit by UV light. Since oxygen blocks UV light, there couldn't have been much oxygen around when these diamonds were made.

Reasoning: Old diamonds contain sulfur-33, which only forms via UV light; however, UV light is blocked by atmospheric oxygen.

Analysis: This stimulus provides a series of conditional facts that lead to a clear deduction. If sulfur-33 is present, then UV light must have reached the surface. If UV light reached the surface, then there could not have been more than a trace of oxygen in the atmosphere. Since the 2.9-billion-year-old diamonds do indeed have high sulfur-33, we can conclude that the atmosphere at that time lacked significant oxygen. The correct answer will likely be a straightforward restatement of this scientific deduction.