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.

[EX Q]: Scientists discovered that although infectious bacteria may develop immunity to antibiotic medications, some bacteria are unable to resist certain peptides, which act as natural antibiotic substances inside the human body. This discovery led to the development of which new technology?  
 (A) artificial substances that mimic human peptides (B) antibiotics that can be absorbed through the skin (C) bacteria that attack antibiotic-resistant organisms (D) antibiotics that adapt in response to bacterial immunity
[EX A]: A

[EX Q]: 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.
[EX A]: D

[EX Q]: When a battery operated train is turned on, it moves along the track. Which best identifies the order of the types of energy used to make the train move?  
 (A) mechanical, chemical, electrical (B) electrical, chemical, mechanical (C) electrical, mechanical, chemical (D) chemical, electrical, mechanical
[EX A]:
D