A graduated slide is illustrated, with $0.1\,\text{mm}$ intervals observed through an eyepiece graticule. Pollen grains were cultivated 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 four 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$10\,\text{mm h}^{-1}$