A pollen grain was observed under a microscope at $\times 500$ magnification. Its image size was measured to be $75\,\text{mm}$. Which calculation would be used to determine the true size of the pollen grain in millimetres?
- A$\frac{75}{500}$
- B$\frac{500}{75}$
- C$\frac{75 \times 1000}{500}$
- D$\frac{500}{75 \times 1000}$