Question

what determines the length of a geologic time period?
○ fossil record
○ material density
○ meteors and rocks
○ radioactive materials

Explanation:

To determine the length of a geologic time period, radioactive materials (through radiometric dating) are used. Radiometric dating relies on the decay of radioactive isotopes to calculate the age of rocks and fossils, which helps define geologic time periods. The fossil record shows the history of life but doesn't determine time period length directly. Material density and meteors/rocks aren't primary determinants of geologic time period length.

Answer:

radioactive materials