The Modern Biology textbook by publishers Holt, Rinehart and Winston includes answer keys in the teacher's edition textbook. Answer keys are also provided online through the publisher's website on the teacher's log-in that offers additional resources to complement the textbook.
As of 2015, the Modern Biology teacher's edition annotated textbook is available on Amazon.com. The publisher's website at Go.HRW.com provides worksheets, study guides with answer keys and links to additional online resources related to the textbook.
If a newer textbook is used, the answer key in older editions may be outdated and the questions changed to reflect updated concepts and questions that are more current.