Andreas wrote the following hypothesis: "if the number of livestock on earth increases, then the amount of methane gas in the atmosphere will increase because cows produce methane gas as a by-product of digestion.”

This is correct. According to the Food and Agriculture Organization of the United Nations, livestock contributes to 16 percent of the annual methane emissions.

Explanation:

Today, our planet is facing extremely serious environmental issues as a result of human activities. One of the major problems is global warming, a phenomenon that is caused by the accumulation of greenhouse gases in the atmosphere. These gases include carbon dioxide, methane, ozone, amongst others.

Methane production comes from the gut of livestock species and it is attributed to the presence of particular microorganisms known as "methanogenic." In simpler words, over 90 percent of methane comes from cows' burps (gases), while less than 10 percent comes from their flatulence. For instance, a cow can produce around 200 liters of methane per day.

So, definitely, if the number of livestock on Earth increases, the amount of methane will consequently increase as well.

i believe it would be the lungs.