The word "one" in paragraph 3 refers  to______.

B

- Hướng dẫn giải

Dẫn chứng: The study examined workers in three  groups:  one that was allowed no breaks, one that was allowed to  do  anything but use the Internet and one that was allowed 10 minutes to use the Internet and Facebook.

=> "one" thay thế cho "a group"