The diagram depicts a stage micrometer with divisions of $0.1\,\text{mm}$, as seen through an eyepiece graticule. Pollen grains were cultured in a sugar solution and observed with the eyepiece graticule. Diagram 1 shows the pollen grains at the beginning. Diagram 2 shows the pollen grains after $4$ hours. What is the growth rate of the pollen tubes?
- A$5\,\mu\text{m h}^{-1}$
- B$10\,\mu\text{m h}^{-1}$
- C$5\,\text{mm h}^{-1}$
- D$10\,\text{mm h}^{-1}$