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.
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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: Tanya investigates which household objects are attracted to magnets. Which will help her make a conclusion?  
 (A) a prediction (B) a hypothesis (C) a list of supplies (D) a table of results

Answer: D


Question: Sandbars form in many rivers because of  
 (A) erosion of cliffs. (B) water flowing into a lake. (C) dams being built by beavers. (D) sediment being moved downstream by water.

Answer:
D