Detailed Instructions: In this task, you are given a sentence. You must find the primary subject in the supplied sentence. The output must be a sub-sequence in the sentence.
See one example below:
Problem: Rope and its Usage
Solution: Rope
Explanation: The word 'Rope' is the key subject here, while Usage is a property of Rope.

Problem: As the rope is laid up its length contracts like a coiled spring, giving it a certain elasticity.
Solution:
its length