Practice 4

A

2. According to the Big Bang Theory, the universe began expanding about 13.7 billion years ago and it is still expanding.

3.Does the universe has an outer edge or is it endless?

4.Scientists predict that intelligent life exists somewhere in the universe, but they do not know exactly where.

5.Mars probes have photographed rocks with water markings on them, yet no water has been found on this planet.

6.We may not be able to communicate with other life forms for we do not know their way of communication.

7.Instead of taking the psychology final exam, we can write a 10-page research paper or we can have give a presentation.

8.I want to write a research paper, yet I do not know how to write it.

9.Three weeks before the end of the term, I had not started my paper, nor have I collected the materials for it.

10.I needed help choosing a topic, so I went to see my professor.

B.

2.The accident at the nuclear power plant at Three Mile Island in the United States created fears about the safety of this energy source, and the disaster at Chernobyl in the former Soviet Union confirmed them.

3.Solar heating systems are economical to operate, but the cost of installation is very high.

4.Energy needs are not going to decrease, nor are energy sources going to increase.

5.Burning fossil fuels causes serious damage to our planet, so we need to develop other sources of energy.

6.Ecologists know that burning fossil fuels causes holes in the ozone layer, yet people continue to do it.

7.Developing nations especially will continue this harmful practice, for they do not have the money to develop "clean" energy sources.

8.All nations of the world must take action, or our children and grandchildren will suffer the consequences.