Read the following passage and mark the letter A, B, C, or D to indicate the answer to each of the question.

Modern civilization is heavily dependent on energy. Without reliable power sources, we couldn’t operate machines, use transportation, communicate via the Internet, or do many other things. Although everyone recognizes the importance of energy, deciding what kind of energy the world should use in the future is not a simple task. The two leading candidates for this role are green energy and nuclear energy. Currently, most of the energy we use is derived from fossil fuels. Although this energy source has got us this far, there are several problems with it. For starters, it is a finite resource which is quickly running out. Some analysts have even estimated that the Earth could run out of coal and oil within the next 50 years. Burning fossil fuels also creates large amount of pollution, which is harmful to the environment. Although fossil fuels still provide the majority of our power, governments are seeking better energy sources to use going forward. Green energy is one option that is becoming increasingly attractive. It includes technologies such as solar, wind, and hydro power. These are seen as clean sources of energy because they cause very little pollution. In addition, they are completely renewable, so there is no danger that it will run out. The major disadvantage with green energy sources is that they are not cheap. The other alternative is nuclear power. Like green energy, it is also a renewable source of power that will not run out, and it also doesn’t produce air pollution. In addition, nuclear power is more reliable than green energy, as it doesn’t depend on sunshine, rain, or wind to operate. The major issues with nuclear power are safety concerns and nuclear waste. Nuclear power plants create a considerable amount of nuclear waste which is extremely hazardous to people’s health. If they happen to have a meltdown, this could destroy the surrounding area for years to come. The recent nuclear disaster in Fukushima, Japan brought this reality into the spotlight once again. In response, Germany announced it would close down eight of its nuclear plants immediately, and close the rest by 2022. Meanwhile, other nations refuse to give up on nuclear power, stating that these disasters are rare. The debate of whether to use green or nuclear power will likely continue for some time. In the end, it is quite possible that both energy sources will be used to fuel our planet. (Adapted from Intensive Reading Comprehension Skills)

The word “it” in paragraph 4 refers to _______ . A. source of power B. alternative power C. green energy D. nuclear power