The picture depicts a single-celled organism named a Paramecium. In the image, the Paramecium measures $6\,\text{cm}$ in length. The magnification is $\times 400$. Which calculation will produce the correct actual length of the Paramecium, in mm?
- A$\frac{6 \times 10}{400}$
- B$\frac{6 \times 1000}{400}$
- C$\frac{400 \times 10}{6}$
- D$\frac{400}{6 \times 1000}$