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Choice (A), 8, appears in the question stem, so this choice would be
a good candidate for elimination if you're short on time, because it could
be a trap answer.
The correct answer is Choice (C):
If all bags must contain at least one marble, then we must have at least
8 marbles to begin with: 1 + 1+ 1 + 1 + 1 + 1+ 1 + 1 = 8.
At most four bags can contain the same number of marbles. To minimize
the number of marbles we use, we'll want to keep 4 bags with just one
marble: 1 + 1+ 1 + 1 + 1 + 1+ 1 + 1 = 8.
If the remaining bags must all have different numbers of marbles, we
just increase each by one, for a total of 18: 1 + 1+ 1 + 1 + 2 + 3+
4 + 5 = 18.
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