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

As recently as 1960, the length of the
meter was defined as the distance between two lines on a specific platinum–
iridium bar stored under controlled conditions in France. This standard was aban-
doned for several reasons, a principal one being that the limited accuracy with
which the separation between the lines on the bar can be determined does not
meet the current requirements of science and technology. In the 1960s and 1970s,
the meter was defined as 1 650 763.73 wavelengths of orange-red light emitted
from a krypton-86 lamp. However, in October 1983, the meter (m) was redefined
...