What is the main topic of this passage?lk

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. 

      In this era of increased global warming and diminishing fossil fuel supplies, we must begin to put a greater priority on harnessing alternative energy sources. Fortunately, there are a number of readily available, renewable resources that are both cost-effective and earth-friendly. Two such resources are solar power and geothermal power. Solar energy, which reaches the earth through sunlight, is so abundant that it could meet the needs of worldwide energy consumption 6,000 times over. And solar energy is easily harnessed through the use of photovoltaic cells that convert sunlight into electricity. In the United States alone, more than 100,000 homes are equipped with solar electric systems in the form of solar panels or solar roof tiles. And in other parts of the world, including many developing countries, the use of solar systems is growing steadily. Another alternative energy source, which is abundant in specific geographical areas, is geothermal power, which creates energy by tapping heat from below the surface of the earth. Hot water and steam that are trapped in underground pools are pumped to the surface and used to run a generator, which produces electricity. Geothermal energy is 50,000 times more abundant than the entire known supply of fossil fuel resources. And as with solar power, the technology needed to utilize geothermal energy is fairly simple. A prime example of effective geothermal use is in Iceland, a region of high geothermal activity, where over 80 percent of private homes are heated by geothermal power. Solar and geothermal energy are just two of a number of promising renewable alternatives to conventional energy sources. The time is long overdue to invest in the development and use of alternative energy on a global scale.

What is the main topic of this passage?

• A. The benefits of solar and wind power over conventional energy sources.
• B. How energy resources are tapped from nature.
• C. Two types of alternative energy sources that should be further utilized.
• D. Examples of the use of energy sources worldwide.