Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: Runoff is likely to cause more erosion if the land is bare. Plants help hold the soil in place. The runoff water in Figure below is brown because it eroded soil from a bare, sloping field. Can you find evidence of erosion by runoff where you live? What should you look for?. Answer: erosion
[A]: Runoff is likely to cause more what if the land is bare?

[Q]: Passage: Dissolved mineral salts wash into the ocean. As ocean water evaporates, it leaves the salts behind. This makes the water saltier. Ocean water is about 3.5 percent salts. The main salt is sodium chloride. Answer: salts
[A]: Concentration of what, the substance left behind when ocean water evaporates, is about 3.5 percent?

[Q]: Passage: A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve O 2 as one reactant. The combustion of hydrogen gas produces water vapor. Answer: oxygen
[A]:
What chemical element is required for a combustion reaction ?