[Q]: What is the missing first step of the following process: - Over time, more sediment piles on top - The resulting heat and pressure transforms the organic layer into a dark and waxy substance known as kerogen - Left alone, the kerogen molecules eventually crack - The molecules break up into shorter and lighter molecules composed almost solely of carbon and hydrogen atoms - It will turn into either petroleum or natural gas -
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[A]: Dead organic material accumulates on the bottom of oceans, riverbeds or swamps, mixing with mud and sand

[Q]: What is the missing first step of the following process: - Food is chewed and mixed with saliva (spit) - Food travels down the esophagus (throat) - Food travels into the stomach - Acid in the stomach breaks the food down further - Food travels into the small intestine - Nutrients from food are absorbed into the blood stream - Food travels into the large intestine - Most of the remaining material is now waste - The waste is eventually removed from the body.
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[A]: Food is put into mouth

[Q]: What is the missing first step of the following process: - Some is reflected back into space - The rest of the energy is absorbed by land and oceans, heating the Earth - Heat radiates from Earth to space - Some of this heat is trapped by greenhouse gases in the atmosphere - Human activities such as burning fuel are increasing the amount of greenhouses gases - This causes more heat radiation to stay trapped in our atmosphere - The Earth's temperature rises -
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[A]: Solar radiation reaches Earth's atmosphere

[Q]: What is the missing first step of the following process: - Air enters bronchial tubes - Air is then split into the bronchioles of each lung - Bronchioles have alveoli which are tiny air sacs - Alveoli is surrounded by many small blood vessels - Oxygen passes through alveoli into blood vessels - Blood leaves the lungs as it travels through the body - Blood carries carbon dioxide back to the lungs - Carbon dioxide released when you exhale.
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[A]:
You breathe air in