A certain radioactive element, call it "X" decays into a stable isotope of another element "Y". The half-life of X is 1 billion years. If a sample has 2 atoms of X for every 30 atoms of Y, which is the best estinate of its age?
a) 4 billion years.
b) 1 billion years.
c) 15 billion years.
d) 32 billion years.
e) There is not enough information given to know.