28. Biogas is and cheap for cooking and heating. A. plenty B. abundant C. serious D. expensive 29. “Why is it called a source?” – “Because it can be replaced easily and quickly.” A. effective B. specific C. renewable D. non-renewable 30. At this time next week, we a wind turbine in our garden. A. install B. installed C. will installed D. will be installing 31. Alternative sources of energy developed in the near future. A. is B. was C. has been D. will be 32. Hydro power is because dams cannot be built in certain areas. A. limited B. unlimited C. abundant D. enough 33. Fossil fuels are very common in our society but they the environment. A. renew B. provide C. waste D. pollute 34. By the middle of the 21st century, people in developing countries more renewable energy. A. uses B. will be using C. used D. have used 35. The wind, the sun, and the wave are some types of sources of energy. A. changeable B. alternative C. cheap D. costly