The unit of length, metre was originally defined as the distance between two fine lines engraved on gold plugs near the ends of a bar of platinum-iridium alloy that is kept `0^@` C. Then the unit was defined in October 1983 in terms of wavelength of kr. 86 radiation .
Now, the present unit is defined in terms of distance travelled by light .
In your opinion, which of the two essential requirements for a standard unit availability, invariability played the key role in this modification.

Explanation

Invariability.