input: Please answer the following: What is the missing first step of the following process: - After a rain, water seeps into cracks and pores of soil and rock - The water percolates beneath the land surface - Some of the water reaches a zone where all the cracks and pores in the rock are already filled with water - Rainwater absorbs some carbon dioxide as it passes through the atmosphere and even more as it drains through soil and decaying vegetation - The water combines chemically with the carbon dioxide - A weak carbonic acid solution is formed - This acid slowly dissolves calcite, forms solution cavities, and excavates passageways -
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output: Solution caves are formed in limestone and similar rocks by the action of water

input: Please answer the following: What is the missing first step of the following process: - The liver uses chemicals and proteins to treat the blood - The blood is cleaned - Energy is produced - The blood leaves the liver cleaned.
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output: Blood is sent to the liver

input: Please answer the following: What is the missing first step of the following process: - People burn coal for heat or electricity - Pollution from the coal goes into the sky - Coal pollution enters clouds - Clouds rain over an area - The polluted rain becomes acid rain.
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output: People dig up coal from the ground

input: Please answer the following: What is the missing first step of the following process: - Ice inside the cloud bumps around and causes electrostatic charges - The cloud becomes a thundercloud - Electrostatic energy builds and forms a strong negative charge - Positive energy is attracted to the negative energy in the cloud - This positive energy gathers on the ground - The positive energy connects with the negative energy and forms lightning.
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output:
Ice forms inside a cloud