Given a scientific question, generate a correct answer to it.
--------
Question: What has traditionally been defined as a force of attraction between two masses?

Answer: gravity

Question: Studies have shown that not only does reproduction have a cost as far as how long male fruit flies live, but also fruit flies that have already mated several times have limited amounts of this?

Answer: sperm

Question: What is light that has only a single wavelength called?

Answer:
monochromatic