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.

Example Input: The cactus moth was deliberately introduced to Australia in 1926 as a biological control for an invasive prickly pear cactus. By the early 1990s the moth had also migrated to the Florida Keys from South America and the moth soon eliminated most of the semaphore cacti in the region. Since then, the moth has adapted to eat other plants in the Keys and has continued to grow in number. Which procedure would most likely reduce the population of the cactus moth?  
 (A) Release sterile moths to limit reproduction. (B) Use widespread spraying of insecticides. (C) Introduce a predator of the cacti. (D) Stop importation of all types of plants.
Example Output: A

Example Input: Nonaggressive birds called purple martins nest each spring to lay eggs. Starlings are aggressive birds that will eat the purple martin's eggs and nestlings. Which will most likely be the effect on both populations after a starling invasion?  
 (A) Purple martins will increase and starlings will decrease. (B) Purple martins will increase and starlings will increase. (C) Purple martins will decrease and starlings will decrease. (D) Purple martins will decrease and starlings will increase.
Example Output: D

Example Input: Two balls rolled down two identical inclined planes. The balls were exactly the same mass and size, but one ball rolled down the inclined plane faster. Identify a possible reason that one ball rolled faster than the other ball.  
 (A) One ball was red and the other ball was blue. (B) One ball was new and the other ball was old. (C) One ball was shiny and the other ball was dull. (D) One ball was sticky and the other ball was smooth.
Example Output:
D