Read the passage and choose the best answer Scientists are now looking (1) ______ cheaper way to use solar energy to heat air and water. Many buildings are using solar energy to heat water now. Solar units on the (2) _______ of buildings collect the energy and use it to heat water. Although these units are still very (3) ________, more and more people are buying solar hot water heaters. Universities and colleges give solar energy courses where students learn to build their own solar (4) ...

Read the passage and choose the best answer

Scientists are now looking (1) ______ cheaper way to use solar energy to heat air and water. Many buildings are using solar energy to heat water now. Solar units on the (2) _______ of buildings collect the energy and use it to heat water. Although these units are still very (3) ________, more and more people are buying solar hot water heaters. Universities and colleges give solar energy courses where students learn to build their own solar (4) ________. In the future, there will be more and more solar collectors, especially in the sunny desert areas of the world.