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.

Q: Which organ in a frog has a function similar to the function of lungs in a bird?  
 (A) kidney (B) skin (C) liver (D) heart

A: B
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Q: Which question will best help a student determine the composition of an unknown substance?  
 (A) Is the sample warm or cold? (B) How valuable is the sample? (C) Has the sample ever been melted? (D) Does the sample conduct electricity?

A: D
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Q: People without safe drinking water may get an illness called dysentery. Dysentery is caused by a type of amoeba that lives inside the body. Doctors can find these amoebas by looking at body samples under a microscope. Which characteristic of amoebas best explains why doctors use microscopes to find dysentery amoebas?  
 (A) Amoebas can change their shape. (B) Amoebas need humans to survive. (C) Amoebas look just like human cells. (D) Amoebas are made of only one cell.

A:
D
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