Question

which unit of geologic time is the oldest?
cenozoic
mesozoic
ordovician
precambrian

Explanation:

The Precambrian is the earliest and longest - lasting division of geologic time. It encompasses the time from the formation of the Earth around 4.6 billion years ago until the beginning of the Cambrian period about 541 million years ago. The Cenozoic is the most recent era, the Mesozoic is the era of dinosaurs, and the Ordovician is a period within the Paleozoic era.

Answer:

Precambrian