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

To address the question regarding the modification of the definition of the meter, we will analyze the two essential requirements for a standard unit: availability and invariability.

### Step-by-Step Solution:

1. **Understanding the Original Definition**:
- The meter was originally defined as the distance between two fine lines engraved on gold plugs at the ends of a platinum-iridium bar kept at 0°C.
- This definition relied on physical objects that could be measured.

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