You will be given a definition of a task first, then some input of the task.
You are given a background paragraph that describes one or more causal or qualitative relationships such as a relationship in economics or a scientific law and a story that makes use of the concepts or the relationship described in the provided paragraph. You are also given a question about the story that requires an understanding of the relationship described in the background paragraph and the story. You need to come up with an answer to the given question; the answer will be a span from either the question or the story. In order to correctly answer the given question, you need to understand the relationship mentioned in the background paragraph and should be able to use it to understand that in the story. Your answer can not consist of any word that is not mentioned in any of these: the background paragraph, the story, or the question. You can directly copy and paste a span from the story or the question while answering the given question.

Background Paragraph: A After a rock forms, nuclei of a radioisotope inside the rock start to decay. As they decay, the amount of the original, or parent, isotope decreases, while the amount of its stable decay product, or daughter isotope, increases. By measuring the relative amounts of parent and daughter isotopes and knowing the rate of decay, scientists can determine how long the parent isotope has been decaying. This provides an estimate of the rock's age.
Story: Gregory is a geologist who went on an excursion to Australia. While hiking in a cave, Gregory found two radioactive rocks containing Uranium. Gregory then brought the rocks back to his lab in Germany to conduct tests. Gregory discovered that rock A had high amounts of the original uranium isotope present, and low amounts of the daughter uranium isotope present. In rock B, however, there were low amounts of the original uranium isotope present and high amounts of the daughter uranium isotope present.
Question: Which rock, rock A or rock B, had been decaying over a longer period of time?
Output:
rock B.