A radio isotope 'X' with a half life $1.4 \times 10^9$ years decays to 'Y' which is stable. A sample of the rock from a cave was found to contain 'X' and 'Y' in the ratio 1 : 7. The age of the rock is :

Question

( A ) $8.40 \times 10^9$ years
( B ) $1.96 \times 10^9$ years
( C ) $4.20 \times 10^9$ years
( D ) $3.92 \times 10^9$ years