Question

Which of the SI standard units is NOT based on a constant of nature?
A) The meter, because the speed of light can vary from place to place.
B) The second, because cesium cannot be found everywhere.
C) The kilogram, because it is based on an arbitrary piece of metal.
D) The kilogram, because the gram is the standard unit.
E) The meter, because it is defined in terms of the speed of light in vacuum, and there is no perfect vacuum anywhere.

Answer

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