What pair of substances would most likely result in the production of a solid when reacting with a base

Both soluble metal salt and carbon dioxide may react with a base to form a solid or what we call a precipitate,

metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

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