And since the wind is free, operational costs are nearly zero (26) a turbine is

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.
Wind is a clean source of renewable energy that produces no air or water pollution. And since the wind is free, operational costs are nearly zero (26)_____ a turbine is erected. Mass production and technology advances are making turbines cheaper, and (27)_____ governments offer tax incentives to spur wind-energy development.
Drawbacks include complaints from (28)_____ that wind turbines are ugly and noisy. The slowly rotating blades can also kill birds and bats, but not nearly as many as cars, Power lines, and high-rise buildings do. The wind is also variable: If it's not blowing, there's no electricity generated.
Nevertheless, the wind energy industry is (29)_____ . Thanks to global efforts to combat climate change, such as the Paris Agreement, renewable energy is seeing a boom in growth, in (30)_____ wind energy has led the way. From 2000 to 2015, cumulative wind capacity around the world increased from 17,000 megawatts to more than 430,000 megawatts. In 2015, China also surpassed the BU in the number of installed wind turbines and continues to lead installation efforts.
(Adapted from nationalgeographic,com)