TOEFL Reading: ETS-TOEFL阅读机经 - XO55VKX80H87651EL$

Which of the following is mentioned in paragraph 4 as a factor in the economic success of the Muscovite principality? A. improvements made to waterways used in trade B. efforts of Muscovite princes to introduce new agricultural methods C. conditions of comparative peace and security on Muscovite lands D. groups of peasants setting land in other areas of Russia