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.

Rot-proofing was discontinued in 1963, because rot-proofed cordage has deteriorated in hot and humid conditions.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Rot-proofing