Question

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

Explanation:

The Precambrian is the earliest and longest - lasting eon of Earth's history, predating the Phanerozoic eon which includes the Paleozoic (where the Ordovician is), Mesozoic, and Cenozoic eras.

Answer:

Pre - cambrian