First eliminate choices C and D since they both mention that fact 1 is crossing over ( which is also referred to as genetic recombination) . So you're left with A and B and the correct answer is B .
The 3.9 billion year old rock has undergone three half-lives (3.9 divided by 1.3 = 3). After 3 half-lives, 12.5% of the potassium-40 remains undecayed.
