Answer:
They are scheduled for deep underground burial
Explanation:
Honestly none of these answers are correct in a sense.
They are no longer radioactive - This one is absolutely false since spent fuel rods remain dangerously radioactive for about for about 10,000 years
They contain only one type of isotope of the fuel (uranium) - There will be some 235 isotopes as well as some 238. Other isotopes might be present as well but only in very small amounts.
They usually remain radioactive for less than 100 years - As mentioned above, spent fuel rods remain dangerously radioactive for about 10,000 years.
They are scheduled for deep underground burial - This answer is probably the best as it it somewhat true. In the 1980's there was a plan launched by the federal government to bury spent fuel below Yucca Mountain in Nevada. However, this project was later called off to think of a better means of containment.
However...
Scandinavia and Finland have already approved an approach known as "deep storage" and have already begun to work on their own sites.
~Hope this helps!~
