Problem: Given the question: What is the missing first step of the following process: - These substances rise into the high atmosphere - They react with water and oxygen - This creates a more acidic rain - The rain with the high acid content falls.
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The answer is:
Chemicals like sulfur dioxide and nitrogen are released into the air

Problem: Given the question: What is the missing first step of the following process: - Pack essential items in your suitcase such as underwear/socks - Pack weather appropriate clothing in the suitcase - Always bring a jacket or two - Always bring one semi-formal outfit for a nice dinner - Zip up the bag and be ready to go.
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The answer is:
Determine the weather where you are going

Problem: Given the question: What is the missing first step of the following process: - The receptors will lock onto a target cell using these chemical receptors on the surface, which docks onto the cell surface - The receptors then go into the cell - The receptors use it like a factory - The receptors take it over and make it produce thousands or in some cases millions of copies of new viruses - The copies come streaming out of the cell - The copies infect other cells to make more viruses, or they escape from the body and infect a new victim -
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The answer is:
The flu virus has receptors, which are like viral Velcro, on the surface of the virus particle