An average mattress contains 2,000,000 dust mites,
To write the scientific notation of the figure, we must move the decimal point to the left to give
[tex]2,000,000\Rightarrow2\times10^6\text{ dust mites}[/tex]
Dust mites are 0.0042mm in length
To write the scientific notation of the figure, we must move the decimal point to the right until we get a significant figure, which gives
[tex]0.0042\Rightarrow4.2\times10^{-3}\operatorname{mm}[/tex]
Hence, the right option is C
