It is possible to date the rocks by comparing the amount of specific radioactive isotopes they contain. Which of the statement is correct in this respect??

A. Older sediment layers have equal amount of these radioactive isotopes as that of the young ones 
B. Older sediment layers have less amount of these radioactive isotopes as that of the young ones. 
C. Older sediment layers have greater amount of these radioactive isotopes as that of the young ones.
D. Older sediment layers did not have these radioactive isotopes