What was the PRIMARY reason for the collapse of the Kievan Rus

answer; /// i do believe that the correct answer is (d) cooperative play; /// good luck

  b.

there are isolated pockets of farming, generally around rivers and oases, which are used for irrigation.

an informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.