【单选题】

Until about four decades ago, crop yields in agricultural systems depended on (1) resources, recycling organic matter, built-in biological control mechanisms and rainfall patterns.Agricultural yields were (2) but stable. Production was (3) by growing more than one crop or variety in space and time in a field as insurance against pest (4) or severe weather. Inputs of nitrogen were (5) by rotating major field crops with legumes. In turn, rotations suppressed insects, weeds and diseases by effectively (6) the life cycles of these pests.A、typical corn belt farmer grew corn (7) with several crops including soybeans, and small grain production was intrinsic to maintain livestock. Most of the labor was done by the family with occasional hired help and no (8) equipment or services were purchased from off4arm sources. In these type of farming systems the link between agriculture and ecology was quite (9) and signs of environment degradation were seldom evident.

But as agriculture modernization (10) the ecology-farming linkage was often broken as ecological principles were (11) . In fact, several agricultural scientists have arrived at a (12) consensus that modem agriculture confronts an environment crisis.A、growing number of people have become concerned about the long-term (13) of existing food production systems.Evidence has shown that (14) the present capital-and-technology-intensive fanning systems have been extremely productive and competitive, they also bring a (15) of economic, environmental and social problems.
Evidence also shows that the very nature of the agricultural structure and prevailing polices have led to this environmental (16) by favoring large farm size, specialized production, crop monocultures and mechanization. Today as more and more farmers are integrated (17) international economies, imperatives to (18) disappear and monocultures are rewarded by economies of scale. In turn, lack of rotations and diversification (19) key self-regulating mechanisms, turning monocultures into highly (20) agro-ecosystems dependent on high chemical inputs.
A、externalB、internalC、exported
D、imported