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 from 35 to 42.

Today we take electricity for granted and perhaps we do not realize just how useful this discovery has been. Steam was the first invention that replaced wind power. It was used to drive engines and was passed through pipes and radiators to warm rooms. Petrol mixed with air was the next invention that provided power. Exploded in a cylinder, it drove a motor engine. Beyond these simple and direct uses, those forms have not much adaptability.

On the other hand, we make use of electricity in thousands of ways. From the powerful voltages that drive our electric trains to the tiny current needed to work a simple calculator, and from the huge electric magnet in steel works that can lift 10 tons to the tiny electric magnet in a doorbell, all are powered by electricity. An electric current can be made with equal ease to heat a huge mass of molten metal in a furnace, or to boil a jug for a cup of coffee.

Other than atomic energy, which has not as yet been harnessed to the full, electricity is the greatest power in the world. It is flexible, and so adaptable for any task for which it is wanted. It travels so easily and with incredible speed along wires or conductors that it can be supplied instantly over vast distances.

To generate electricity, huge turbines or generators must be turned. In Australia they use coal or water to drive this machinery. When dams are built, falling water is used to drive the turbines without polluting the atmosphere with smoke from coal.

Atomic power is used in several countries but there is always the fear of an accident. A tragedy once occurred at Chernobyl, in Ukraine, at an atomic power plant used to make electricity. The reactor leaked, which caused many deaths through radiation.

Now scientists are examining new ways of creating electricity without harmful effects to the environment. They may harness the tides as they flow in and out of bays. Most importantly, they hope to trap sunlight more efficiently. We do use solar heaters for swimming pools but as yet improvement in the capacity of the solar cells to create more current is necessary. When this happens, electric cars will be viable and the world will rid itself of the toxic gases given off by trucks and cars that burn fossil fuels

The word "they" in the last paragraph refers to ____________.