The fact that United States slavery and Russian serfdom were abolished during the same decade is cited by the author in the first paragraph primarily in order to A. emphasize that rebellions in both countries eventually led to the demise of the two institutions B. cite a coincidence that de Tocqueville should have been able to foresee C. suggest one reason why more historians should have been drawn to a comparative study of the two institutions D. cite a coincidence that Kolchin's study has failed to explain adequately E. emphasize the underlying similarities between the two institutions