D
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
The radioactive isotope content of a rock can be used to identify which property of the rock?

### Choices:
A. the total mass of the rock
B. the rate at which the rock formed
C. the types of fossils that the rock contains
D. the amount of time passed since the rock formed

### Answer:
