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AUSTIN (AP) — The publisher of a high school textbook that referred to slaves brought to America as “workers” is apologizing and says more diversity will be added to review boards.

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McGraw-Hill Education CEO David Levin said Monday in a letter to employees that the company made a mistake. He says slavery “was a horrible part of American history” and that the publisher will provide teachers a free supplemental lesson plan about the Atlantic slave trade.

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The company has been under fire since a Texas mother last week posted her disbelief on social media about the wording in her 15-year-old son’s world geography textbook.

Texas officials say roughly a quarter of the state’s 1,200 school districts are using digital or physical versions of the book in classrooms this year.

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