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He has been working ________ several years on this theory.

  • a) off and on
  • b) on and off

It sounds to me ridiculous but my book says it should be "off and on". Aren't they interchangeable? And if not, what are the nuances?

Thanks in advance.

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They are interchangeable, and this ngram view suggests that on and off is currently about three times as common as off and on. Off and on was a bit more common before the beginning of the 20th century. So perhaps your book is very old.

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