You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

Let me give you an example: Which event can cause a large rock to become soil over time?
(A) decaying (B) an earthquake (C) a landslide (D) weathering
The answer to this example can be: D
Here is why: Under the action of heat, cold, rain, wind, and other atmospheric factors, the rock breaks down physically into small fragments that become the parent material of the soil. Also, atmospheric factors change according to weather.

OK. solve this:
Two students each carry a box up to the third floor of a building. The total mass of each student and the box she is carrying is the same. Roberta makes the trip in 45 s while Mary takes 30 s. Which of the following statements best describes this task?
(A) Roberta does more total work than Mary. (B) Mary does more total work than Roberta. (C) Roberta has a greater power output than Mary. (D) Mary has a greater power output than Roberta.
Answer:
D