Write the correct form of the words in the brackets

Today, we get a lot of energy from fossil fuels: coal, oil, and (17) __[color1]________ gas. But when we burn fossil fuels, they cause air (18) __[color2]________, and they are (19) __[color3]________. This means they will not last forever. In the long term, we will have to find alternative sources of energy. (20) ___[color4]_______ energy uses panels to catch sunlight and convert it into electricity. However, it doesn’t work at night and needs a lot of space. Wind power is another type of (21) __[color5]________ energy source. It’s cheap and produces clean energy, but wind turbines can be (22) ____[color6]______ and some people think they look ugly.