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 on two different school campuses are comparing the growth rate of grass three weeks after fertilizer has been applied. The same fertilizer and the same amount of water are used on both campuses. Which additional variable is most important to control when the results of the two investigations are compared?
type of grass used
Sunlight exposure.