Solar energy is a long lasting source of energy, and it can (21)________almost anywhere. To generate solar energy, we only need solar cells and (22)________! Solar cells can easily be installed on house (23)________, so we don’t need any new space. Compared to other renewable sources, they also possess many advantages: wind and water power rely on turbines which (24)________noisy, expensive and which take up large space. Solar cells are totally silent and (25)________.As they have no moving parts, they require little maintenance and have a long lifetime. 21. A. be used B. is used C. be using D. to be used 22. A. the earth B. the planet C. the sun D. the moon 23. A. yards B. roofs C. gardens D. doors 24. A. is B. be C. are D. was 25. A. polluted B. polluting C. non-polluted D. non-polluting