Which sentence correctly uses a comma to separate the introductory phrase?

I think is the 3rd

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answer: d.she abandons people when they least expect it.

in the monk's tale, we encounter seventeen stories that relate historical events. all of these stories have tragic commonalities. they tell the stories of heroes who, despite their success in life, all met sad ends. the monk blames these sad outcomes on fortune. one of fortune's traits is that she abandons people when they least expect it. this is one of the most common themes of medieval tragedies.

well if u read the book you would know that she meant to