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

Q: Passage: Improving crops by selecting for certain genetic traits. The desired traits promote productivity. Recently, genetically engineered crops have been introduced. Answer: promotes productivity

A: What is the advantage of selecting for certain genetic traits for crops?
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Q: Passage: The distribution of darts on a dartboard shows the difference between accuracy and precision. Answer: accuracy and precision

A: What does the distribution of darts on a dartboard shows the difference between?
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Q: Passage: Electron Configurations of Ions We have seen that ions are formed when atoms gain or lose electrons. A cation (positively charged ion) forms when one or more electrons are removed from a parent atom. For main group elements, the electrons that were added last are the first electrons removed. For transition metals and inner transition metals, however, electrons in the s orbital are easier to remove than the d or f electrons, and so the highest ns electrons are lost, and then the (n 1)d or (n 2)f electrons are removed. An anion (negatively charged ion) forms when one or more electrons are added to a parent atom. The added electrons fill in the order predicted by the Aufbau principle. Answer: electrons

A:
Ions are formed when atoms gain or lose what?
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