Which term refers to the calendar scientists use to outline Earth's history?

• A. Chronological
• B. Evidence
• C. Observations
• D. Geological Time Scale

Answer: (D)

Think of Earth’s long history as a timeline that scientists organize with a special calendar—the geological time scale. This system lays out time from the formation of Earth about 4.6 billion years ago into units like eons, eras, periods, and epochs, each tied to major events such as the beginnings of life, large climate shifts, or mass extinctions. Fossils, rock layers, and dating methods help scientists decide where to place these boundaries and name the units, so everyone can talk clearly about when things happened.

The other terms aren’t the calendar itself. Chronological just means arranging events in time, which is part of using a time scale but doesn’t define the calendar system. Evidence and observations are the data scientists collect to support ideas about Earth’s history, not the calendar used to outline it.