In the fourth paragraph, the word which refers to .

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each ofthe questionsfrom 31 to 35.
Sunlight is solar energy. Sunlight is needed for growing plants that you eat to get energy. Sunlight is also used to make clean electricity. Burning fossil fuels to make electricity pollutes our atmosphere and rivers. Fossil fuels are expensive and limited sources of energy.
Photovoltaic, or PV system use a type of material that converts sunlight into electricity. PV systems can power your air conditioner or a satellite like the International Space Station PV systems are also used to run a calculator, recharge cell phone, or even power lightweight cars.
Sunlight is also changed into electricity by concentrating solar power or CSP systems. CSP systems have mirrors that focus the sunlight. The concentrated sunlight turns water into steam which turns a turbine that is connected to an electric generator. A CSP system usually uses sunlight to make steam. CSP power plants can store large amount of heat.
Sunlight provides the energy needed to grow plants and make large amount of environmentally friendly heat and electricity. Solar energy can provide power today and for a long time in the future.