Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: The use of nuclear fusion for energy has several pros. Unlike nuclear fission, which involves dangerous radioactive elements, nuclear fusion involves just hydrogen and helium. These elements are harmless. Hydrogen is also very plentiful. There is a huge amount of hydrogen in ocean water. The hydrogen in just a gallon of water could produce as much energy by nuclear fusion as burning 1,140 liters (300 gallons) of gasoline! The hydrogen in the oceans would generate enough energy to supply all the world's people for a very long time. Answer: nuclear fusion
Hydrogen and helium are specifically involved as reactants in what nuclear reaction that is useful for making energy?