Given a scientific passage and an answer, generate a question for the given answer.

Q: Passage: Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons. Answer: aphotic zone

A: What do you call the zone in a body of water where there is too little sunlight for photosynthesis?
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Q: Passage: The force of gravity is called weight. Near the surface of a planet, it has magnitude and is directed perpendicular to its surface. This is different from the Gravitational Constant, and differs from planet to planet. Answer: weight

A: What is the force of gravity called?
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Q: Passage: An embryo is an organism in the earliest stages of development. Embryos of different species may look quite similar, even when the adult forms look very different. Look at the drawings of embryos in Figure below . They represent very early life stages of a chicken, turtle, pig, and human being. The embryos look so similar that it's hard to tell them apart. Such similarities provide evidence that all four types of animals are related. They help document that evolution has occurred. Answer: an embryo

A:
What is defined as an organism in the earliest stages of development?
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