What is a double displacement reaction?

A double displacement reaction is a type of chemical reaction where two compounds react, and positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

Na2S+2HCl→2NaCl+H2S

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answer

i think the answer is a) as air pressure in an area increases, the density of the gas particles in that area decreases.

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c, liquids evaporate, creating a gas