Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: 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?
The container without the onion is the control.
The onion was already present in the first three containers.