What antecedent refers to "their"

Which of these authors wrote a story that revolved around a captains underwater adventures with his crew and a sub marine long before the submarine was actually invented

Before ending this lesson, take a moment to review an informative research essay you have written. you’ve probably already written a conclusion to the essay. but is it the best conclusion it can be? does it properly summarize the main subtopics of your essay? does it state in one or two sentences the “so what” of your topic. if your topic was abraham lincoln, your essay shouldn’t be just about abraham lincoln. it should answer, “so what about abraham lincoln? ” take five or ten minutes, and looking only at that single paragraph, revise your conclusion

What is the answer to this question