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

[Q]: Passage: - For lenses, the distance from the center of the lens to the focus is . Focal lengths are positive for converging lens and negative for diverging lens. The distance from the center of the lens to the object in question is , where distances to the left of the lens are positive in sign. The distance from the center of the lens to the image is . This number is positive for real images (formed to the right of the lens), and negative for virtual images (formed to the left of the lens). Answer: focal lengths
[A]: What lengths are positive for converging lens and negative for diverging lens?

[Q]: Passage: Gases are also capable of dissolving in liquids. There are many examples of this in our everyday lives. For example, carbonated beverages contain dissolved carbon dioxide. We notice this when bubbles come out of solution when the beverage is opened. Another example is when oxygen from the air we breathe dissolves in our blood, where it is transported throughout the body. Fish and other aquatic organisms use gills to capture dissolved oxygen from their environments. Answer: oxygen
[A]: Fish and other aquatic organisms use gills to capture dissolved what?

[Q]: Passage: The basic unit of matter is an atom . At the center of an atom is its nucleus . Protons are positively charged particles in the nucleus. Also in the nucleus are neutrons with no electrical charge. Orbiting the nucleus are tiny electrons. Electrons are negatively charged. An atom with the same number of protons and electrons is electrically neutral. If the atom has more or less electrons to protons it is called an ion . An ion will have positive charge if it has more protons than electrons. It will have negative charge if it has more electrons than protons. Answer: atoms
[A]:
What is the basic unit of matter?