Sustainable development is applied to just

问题：

Sustainable development is applied to just about everything from energy to clean water and economic growth, and as a result it has become difficult to question either the basic assumptions behind it or the way the concept is put to use. This is especially true in agriculture, where sustainable development is often taken as the sole measure of progress without a proper appreciation of historical and cultural perspectives.
To start with, it is important to remember that the nature of agriculture has changed markedly throughout history, and will continue to do so. Medieval agriculture in northern Europe fed, clothed and sheltered a predominantly rural society with a much lower population density than it is today. It had minimal effect on biodiversity, and any pollution it caused was typically localized. In terms of energy use and the nutrients captured in the product it was relatively inefficient.
Contrast this with farming since the start of the industrial revolution. Competition from overseas led farmers to specialize and increase yields. Throughout this period food became cheaper, safer and more reliable. However, these changes have also led to habitat loss.
What’s more, demand for animal products in developing countries is growing so fast that meeting it will require an extra 300 million tons of grain a year by 2050. Yet the growth of cities and industry is reducing the amount of water available for agriculture in many regions.
All this means that agriculture in the 21st century will have to be very different from how it was in the 20th. This will require radical thinking. For example, we need to move away from the idea that traditional practices are inevitably more sustainable than new ones. We also need to abandon the notion that agriculture can be "zero impact". The key will be to abandon the rather simple and static measures of sustainability, which centre on the need to maintain production without increasing damage. Instead we need a more dynamic interpretation, one that looks at the pros and cons of all the various ways land is used. There are many different agricultural performances besides food yield: energy use, environmental costs, water purity, carbon footprint and biodiversity. It is clear, for example, that the carbon of transporting tomatoes from Spain to the UK is less than that of producing them in the UK with additional heating and lighting. But we do not know whether lower carbon footprints will always be better for biodiversity. What is crucial is recognizing that sustainable agriculture is not just about sustainable food production.

What is the author’s purpose in writing this passage

A. To remind people of the need of sustainable development.
B. To suggest ways of ensuring sustainable food production.
C. To advance new criteria for measuring farming progress.
D. To urge people to rethink what sustainable agriculture is.