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

### Question:
What should a scientist do to help make sure the results of an experiment are unbiased?

### Choices:
A. Keep the results a secret.
B. Do the math without a calculator.
C. Change all of the variables in the experiment.
D. Ask other scientists to attempt to reproduce the results.

### Answer:
