Which of the Following Best Matches the Description

Which of the following best matches the description?
-Naturally occurring radioactive isotope used in nuclear reactors.

A) thermal pollution
B) alpha radiation
C) nuclear fission
D) light-water reactor
E) nuclear breeder reactor
F) radiation
G) beta radiation
H) decommissioning
I) none of above
J) radioactive half-life
K) nuclear fusion
L) heavy-water reactor
M) gamma radiation
N) moderator
O) rem
P) U-235
Q) transuranic waste