Directions (For the following 21 items) :

In this section there are six short passages. Each passage is followed by items based on the passage. Read each passage and answer the items that follow.

To what extent, though, are modern farming methods 'sustainable'? There is abundant evidence that a high price has to be paid to sustain the high rates of food production achieved by farmed monocultures. For example, they offer ideal conditions for the epidemic spread of diseases such as mastitis, brucellosis and swine fever among livestock and coccidiosis among poultry. Farmed animals are normally kept at densities far higher than their species would meet in nature with the result that disease transmission rates are magnified. In addition, high rates of transmission between herds occur as animals are sold from one farming enterprise to another; and it is easy for the farmers themselves, with mud on their boots and their vehicles, to act as vectors of pests and disease.

What is the essence of this passage?