Question - Fingernail growth rates

Question: Making some basic assumptions, how fast do your fingernails grow?

Answers:
A. 1*10^-3 m/s
B. 1*10^-6 m/s
C. 1*10^-9 m/s
D. 1*10^-12 m/s

How to work it out:

First, make those 'basic assumptions' mentioned earlier in the question. How fast do my fingernails grow? I assumed 5cm/year. As it turns out, I wasn't too far off, as Wikipedia suggests 3.5 cm. I then made sure I had this result in the correct units - first adjusting cm to m, and then years to seconds. You make need to make sure that you are confident with scientific notation. This estimate brought me fairly close to the result of 1*10^-9m/s - option C.