A student measures the thickness of a human hair by looking at it through a microscope of magnification 100. He makes 20 observations and finds that the average width of the hair is $3.5 \mathrm{mm}$. What is thickness of the hair?

• A

  0.0035 mm

• B

  0.035 mm

• C

  0.01 mm

• D

  0.7 mm