The author cites which one of the following as a factor that might have discouraged historians from undertaking a comparative study of Russian serfdom and United States slavery? A. major differences in the political systems of the two countries B. major differences in the demographics of the two countries C. the failure of de Tocqueville to address the subject D. differences in the size of the estates on which slaves and serfs labored E. the comprehensiveness of Kolchin's own work