Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.

        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. Nuclear fission is used to create enormous amount of heat and electricity. However, nuclear fission forms dangerous radioactive waste.

          Sunlight offers many ways to get energy. A window can allow warm sunlight into your room. Solar water-heating systems can use sunlight to warm the water for your home, swimming pool and school. Sunlight also warms the Earth and causes wind. Electricity can be made by wind generators.

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. The stored heat is used to make electricity at night. During cloud days, many CSP plants can also burn natural gas to provide the heat that is used to turn water into steam.

         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.

In the fourth paragraph, the word “which” refers to _______