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: Certain flowers will open for twelve hours and then close. Which stimulus do the flowers respond to when they open and then when they close for that time period?
light levels
Light (opens), Darkness (closes).