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.

Example input: Rope and its Usage
Example output: Rope
Example explanation: The word 'Rope' is the key subject here, while Usage is a property of Rope.
Q: When mature the abaca plant, which grows to a height of from 10 to 30 ft, is felled and the fibre from the leaf sheaths is stripped off.
A:
the abaca plant