For each scenario below, indicate whether a chemical reaction or a nuclear reaction is occurring.
a. When two compounds are combined heat is released.
b. A sample of galium-68 is reduced to one-half of its original amount in 68.3 minutes.
c. Radium-226 decays to radon-222.
d. A spark of energy is used to begin the combustion of methane gas.
e. Hydrogen nuclei are fused in the Sun to make helium atoms​