Final answer:
The correct sentence is: They said their goodbyes, for they knew it was time.
Step-by-step explanation:
The correct sentence is: They said their goodbyes, for they knew it was time. The comma is used to separate the two independent clauses 'They said their goodbyes' and 'they knew it was time.' The word 'for' is used to show the reason or cause for saying their goodbyes. So, the sentence is correct grammatically.
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