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

### Question:
Which best describes how ice cores are important to the study of geologic history?

### Choices:
A. They show unconformities, which signal changes in deposition.
B. They hold index fossils, which are used to date the different ice cores.
C. They contain evidence showing changes in the atmospheric composition over time.
D. They follow the Law of Superposition, which gives reasons for extinctions of species.

### Answer:
