Radioactive decay occurs when which of the following happens?

two nuclei combine together

an isotope spontaneously breaks down and releases particles

isotopes of uranium are used to to make power

Question

Radioactive decay occurs when which of the following happens?
		
two nuclei combine together
		
an isotope spontaneously breaks down and releases particles
		
isotopes of uranium are used to to make power

anonymous · Answer

Well,nuclear fusion doesn't give off ray

anonymous · Answer

So which of the 3 could it be ?

anonymous · Answer

it appears to me that 

an isotope spontaneously breaks down and releases particles
		
isotopes of uranium are used to to make power