You will be given a definition of a task first, then some input of the task.
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.

A group wants to find out if the number of water birds flying through Arkansas changes each year. Which method should they use to gather this information?
(A) Catch as many birds as possible one year. Collect data on the location of each bird. (B) On the first day of spring next year, count the number of birds at the biggest lake in the state. Compare the data to last year. (C) Put an observer at a location on the state line. Have that person count the birds as they fly into the state. Do this for two years. (D) Pick one day a year during the migration. Count the number of birds at ten different lakes across the state. Do this on the same day every year for ten years.
Output:
D