Given a scientific passage and an answer, generate a question for the given answer.
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Question: Passage: Oxygen is needed to make ozone. Ozone is a molecule made of three oxygen ions. Ozone in the upper atmosphere blocks harmful solar radiation from reaching the surface. Without oxygen, life on Earth would have been very simple. Answer: three

Answer: How many oxygen ions make up an ozone molecule?

Question: Passage: Pea plants are naturally self-pollinating. In self-pollination , pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is called cross-pollination . The offspring that result from such a cross are called hybrids. Answer: cross-pollination

Answer: What is it called when pollen from one plant fertilizes another plant of the same species?

Question: Passage: Pipettes are small, but important tools in many biology labs. Micropipettes, such as the ones shown here, are calibrated to measure very small amounts of liquids. For example, 100 microliters (100 L) is about half the volume of your little finger tip; or even 1 L, which is much smaller than a drop of water. Answer: micropipettes

Answer:
What kind of pipettes are suitable for measuring very small amounts of liquids?