The fertile soil of Mexico's plateaus results from A. ash deposited by volcanoes
B. artificial fertilizers
C. decaying vegetation
D. the flooding of rivers

Ibelieve that the answer is (a).

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.