A、One. B、Two. C、Three. D、Four. C

Solar cells convert energy from the sun into electricity. The researchers have replaced a toxic compound, used to make one type of solar cell, with a chemical that is much cheaper, completely safe and works just as well.
   The new compound is used to make tofu and is found in bath salts. It’s also found in sea water, and so costs much less than the poisonous chemical currently used.
   Dr. Jon Major, who led the research at Liverpool University, believes that "this process could really make the difference into making it competitive with fossil fuels."
   More work will need to be done to see if the cost savings found in the lab can work on an industrial scale. But the cost of solar energy has been steadily falling. And many involved in research in the field believe that it’s just a matter of time before it becomes cheaper than coal, gas and oil, and one day replaces fossil fuels entirely.
3. What’s the advantage of the new chemical according to the news?
4. How many fossil fuels have been mentioned in this news?