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.

Scientists are now looking (1) ______ cheaper way to use solar energy to heat air and water.