A bacterium cell swims by rotating its bundle of flagella to counter a viscous drag force in the medium. The drag force F(R, v, \eta) only depends on the typical length scale of the cell R, its speed v, and the viscosity of the fluid \eta, which has units of \mathrm{kg} /(\mathrm{m} \cdot \mathrm{s}). It is observed under a microscope that a cell of length 1 \mu \mathrm{m} swims at about 20 \mu \mathrm{m} / \mathrm{s}. Estimate the speed of a cell of length 0.5 \mu \mathrm{m}, assuming cells of all sizes generate the same amount of force from their flagella.
Answer Choices
A. 5 \mu \mathrm{m} / \mathrm{s}
B. 10 \mu \mathrm{m} / \mathrm{s}
C. 40 \mu \mathrm{m} / \mathrm{s} \leftarrow CORRECT
D. 80 \mu \mathrm{m} / \mathrm{s}
E. There is not enough information to decide.