Ultraviolet rays are used to
kill cancer cells through radiation therapy
grow plants wear little light is available
keep food warm for extended periods of time
scan packages and baggage for dangerous content

Grow plants where little light is available

Explanation:

The plants need the ultraviolet rays in order to be able to survive and develop. The need mainly comes from the dependence of these rays for production of food, in a process known as photosynthesis. The plants are producers, thus they create their own food. In order to be able to do that they are using the ultraviolet rays, as well as water, and carbon dioxide. By combining them, the plants manage to create glucose for them, and that is their food source. The plants that are kept at places where there's not enough light are often exposed to ultraviolet rays so that they are able to perform the process of photosynthesis and grow properly.

a.) travels faster in solids because the particles are closer together.

air is a gas water is a liquid