The Cambrian Explosion is best described as a period of:

• A. Rapid evolution and diversification of complex life
• B. Global glacier advance
• C. Sudden oceans boiling away
• D. Stable climate with no life changes

Answer: (A)

The key idea being tested is that the Cambrian period is characterized by a rapid burst of evolution and the emergence of many complex, multicellular life forms. In a relatively short span of geologic time, a wide variety of animal life—many with hard shells and distinct body plans—appeared in the fossil record. This reflects a real acceleration in evolutionary rate and ecological innovation, not a static era. Factors such as rising oxygen levels and new ecological interactions (like predator–prey relationships) helped drive this diversification, making the period famous for the sudden appearance of many major animal groups.

Other options don’t fit because they describe climate or environmental states that aren’t what the Cambrian is known for. Glacial advances imply widespread cooling and ice sheets, which aren’t the defining feature of this period. The idea of oceans boiling away would indicate extreme greenhouse conditions not associated with the Cambrian fossil record. And a stable climate with no life changes contradicts the substantial and rapid diversification seen in fossils from this time.