Because they do not have the same qualities therefore they are different

<em>Chemists use the mole unit to represent 6.022 × 10 23 things, whether the things are atoms of elements or molecules of compounds. This number, called Avogadro's number, is important because this number of atoms or molecules has the same mass in grams as one atom or molecule has in atomic mass units. </em>
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B is your best answer because a mixture is when two or more things combine but not chemically. Take soup for example you take out all the pieces because they didn't combine together and just become 1 thing they still have parts. You can still take the noodles, you take the fish or meat out still, you take the broth away to.