Question:
Dogs are friendlier than cats since they would rather be alone.

Who is they referring to in this sentence?
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Answer:
cats

Question:
American Airlines offers upgrades for their preferred passengers so they continue choosing their airlines over others.

Who is they referring to in this sentence?
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Answer:
their preferred passengers

Question:
Nancy adopted a baby girl because she always wanted a daughter.

Who is she referring to in this sentence?
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Answer:
Nancy

Question:
Mary cleaned Susan's room and she was thankful.

Who is she referring to in this sentence?
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Answer:
Susan