If a scientist wants to figure out how old a rock layer is, would it be useful to use a substance that takes a long time to break down (has a long half life), or a substance that breaks down quickly? is it A. A substance that breaks down slowly, because then there won't be enough left to measure

B. A substance that breaks down slowly, because then there will be enough left to measure.

C. A substance that breaks down quickly, so it will be all gone and we'll know it's old