Passage Summary: When ice caps get bigger, a certain type of oxygen increases in the ocean. Shellfish data shows this oxygen increased for 20,000 years and then decreased for 10,000 years.

Reasoning: Heavy oxygen levels in the sea rise when ice caps grow and fall when they shrink; shellfish records show these levels rose for 20,000 years and then fell for 10,000 years.

Analysis: This question requires you to synthesize two rules to reach a logical deduction. We are told that heavy oxygen in the sea is a direct indicator of ice cap size: more oxygen means more ice, and less oxygen means less ice. Since the shellfish (which live in the sea) showed an increase in this oxygen for 20,000 years, we can conclude the ice caps were growing during that time. Conversely, the subsequent 10,000-year decrease indicates the ice caps were shrinking. Look for an answer that simply mirrors this timeline of ice cap growth followed by recession.