Please choose the correct ending to complete the given sentence: Health: [header] How to get vitamin d naturally [title] Learn how the sun creates vitamin d. [step] Sunlight is a form of radiation that has two forms, ultraviolet a (uva) and ultraviolet b (uvb) radiation. Your main source of vitamin d occurs when uvb radiation strikes your skin. When this happens, the first steps of vitamin d synthesis in your body is triggered.

Ending1: In addition, the most powerful blue rays that cause vitamin d are found in the sun. Your uvb radiation travels through your entire body including your skin and gets to the spot where you are prone to sunburn. Ending2: [substeps] Your skin reacts directly to uvb rays. You don't get the energy necessary to take any " sun-enhancing " steps yet. Ending3: After this, the next steps occur in the liver and then the kidneys. [substeps] Because the rest takes place inside the body, once the body is triggered, you no longer need to be exposed to uvb radiation. Ending4: [substeps] Ultraviolet a (uva) is the active radiation that is produced by the sun, but is not useful for biosynthesis. Instead, vitamin d is produced on the uva inner layer of your skin.

Answer format: ending1/ending2/ending3/ending4