Complete each sentence with two words. (Hoàn thành mỗi câu với hai từ.) 1. … are limited and cause pollution. 2. … is not new to us because we use it every day. 3. … catch the sunrays and change them into electricity. 4. In the past, sailors used … to sail their ships to different areas. 5. Both solar energy and wind power are … sources of energy.