A student sketches a red blood cell. The diameter of the red blood cell in the sketch is $20\,\text{mm}$. The true diameter of the red blood cell is $0.008\,\text{mm}$. What magnification has been used for the student’s sketch?
- A$\times 0.0004$
- B$\times 0.16$
- C$\times 250$
- D$\times 2500$