A、By using satellite images, maps, etc. B、By studying specific farming methods. C、By studying the variation of human population.

Experts say some farming activities are seriously damaging Earth’s environment. Bad farming methods can damage soil, forests and water supplies. They also have caused some plants and animals to disappear.
    Two groups now say that bad farming methods are threatening the ability of farmers to produce enough food to feed the world population in the future.
    The groups used satellite images, maps and modern equipment to study the effect of agriculture on the environment. The study found that the destruction of soil is widespread. Important organic nutrients are being removed from farmland.
    Agriculture uses seventy percent of the freshwater in the world each year. The study found that many farmers are wasting water supplies. In addition, chemicals designed to protect crops may pollute waterways.
    The report also says as much as thirty percent of the world’s forests have been cut down so the land could be used for agriculture. This has led to a severe loss of wildlife and their environments.
Questions:
29. Which is not mentioned in the passage as an aspect that farming activities are damaging?
30. How can the group carry out their studies?
31. According to this passage, why can the farming activities affect the animals?