Why the answer is B, and why the others tempt you.
The SI prefix 'nano' (symbol: n) represents a factor of $10^{-9}$. For example, 1 nanometre (nm) = $10^{-9}$ m. Micro ($10^{-6}$), pico ($10^{-12}$), and milli ($10^{-3}$) are different prefixes.
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