The Reason We Know the Exact Length Corresponding to One

The reason we know the exact length corresponding to one meter today is because

A) our cars can calculate distance traveled with highly calibrated odometers
B) there is a metal rod exactly one meter long, kept at the National Bureau of Standards
C) we know the exact distance in meters from the Earth to the Moon
D) we can use the distance from the Queen of England's nose to her finger tips as the standard of length measurement
Fill in the Blank Questions