Mark the letter A, B, C, or D on your answer sheet to indicate the correct option that best fits each of the numbered blanks.

Seventy percent of people in Viet Nam live in rural areas. Most of these people (19) ____ their living by caring for farm animals like pigs. Other common jobs are crop farming and fishing. For many of these farmers and fishermen, animal and plant waste is still a problem.

Many women and children cook meals using traditional cooking fuels. They also spend many hours (20) ____ wood and other materials to burn. This process takes time and also uses a lot of natural resources. Moreover, they suffer the same health concerns. Cooking was a difficult and dangerous job. The kitchen was smoky, and the black ash would make people cough, and (21) _____ pain in the eyes. But a biogas system can (22) ____ all of these problems! It can turn animal or plant waste into valuable resources such as cooking fuel, electricity and plant fertilizer. First, an expert must build a system of pipes and tanks. This biogas system contains and manages the decaying process of animal or plant waste. To use the biogas system, a fanner must move animal waste into a pipe with water. These pipes bring the waste to a tank buried in the ground. This tank is called a bio-digester. Air cannot enter this tank. In this contained environment, natural bacteria decay the waste material. (23) ____ the waste decays in the bio-digester, it (24) ____ two things: biogas and slurry. The biogas rises to the top of the tank. The pressure from the gas pushes the liquid slurry into another tank. Now people can use both the biogas and the slurry.