In the described serial dilution, what is the dilution factor per step?

• A. 2
• B. 5
• C. 10
• D. 20

Answer: (C)

In a serial dilution, the key idea is the dilution factor per step—the ratio by which the concentration is reduced in that single step. If you transfer a small amount of sample into enough diluent to make the total volume ten times the aliquot, you get a tenfold dilution: the concentration is reduced by a factor of 10 in that step. This is why the correct choice is 10. Other factors would come from using different transfer-to-diluent ratios—for example, a 1:1 transfer would be a twofold dilution, a 1:4 setup a fivefold dilution, and a 1:19 setup a twentyfold dilution.