The half life of a radioactive isotope 'X' is 20 years .It decades to another element 'Y' is 20 years . It decays to another element 'Y' which is stable . The two elements 'X' and 'Y' were found to be in the ratio 1:7 in a sample of a given rock.The age of the rock is estimated to be:

A

100 years

B

40 years

C

60 years

D

80 years