Question

Which unit of geologic time is the oldest?

Cenozoic
Mesozoic
Ordovician
Precambrian

Answer 1

The oldest unit of geologic time among the options provided is the **Precambrian**. The Precambrian encompasses the vast majority of Earth's history, starting from the formation of the Earth about 4.6 billion years ago up until approximately 541 million years ago, when the Paleozoic Era (which includes the Ordovician period) began.