instruction:
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.
question:
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.
answer:
D

question:
Several species of fish live in a classroom aquarium. Students notice that the gills of one type of fish move faster than the gills of the other fish. Which of these statements is the best hypothesis on this observation?
(A) Fish have a two-chambered heart. (B) Some fish are much larger than others. (C) Fish need different amounts of oxygen. (D) There are six types of fish in the aquarium.
answer:
C

question:
Earth has a definite orbit within the solar system. This orbit is mainly a result of the
(A) mass of the Sun. (B) size of the Moon. (C) pulling force of other planets. (D) distance between other planets.
answer:
A