Size of specimens — practice question

A bacterial cell appears with a length of $40\,\text{mm}$ under a magnification of $\times 20\,000$. What is the cell’s true length?

• A$0.0002\,\text{mm}$
• B$0.002\,\text{mm}$
• C$0.02\,\text{mm}$
• D$0.2\,\text{mm}$