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

### Question:
Students conducted an investigation with green bananas. They hypothesized that an onion placed near a banana could increase the rate at which the banana ripened. They placed one onion and one banana in each of three containers. In a fourth container they placed a banana, but no onion. Which statement best explains why the onion was left out of the fourth container?

### Choices:
A. The onion would contaminate the fourth trial.
B. The container without the onion is the control.
C. Three containers with onions are enough to conduct the test.
D. The container without the onion is the independent variable.

### Answer:
