Answer the following question given this paragraph: 

The use of nuclear fusion for energy has several pros. Unlike nuclear fission, which involves dangerous radioisotopes, nuclear fusion involves 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.


Q: Unlike nuclear fission, which involves dangerous radioisotopes, nuclear fusion involves what two harmless elements?


A:
hydrogen and helium