The choices are definitely narrowed down to A) and C). The problem arises in choosing which among st the two.
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Lets have a look at
A) The prices have fallen dramatically - For this to happen, means, that oil has become cheaper than it already was (it is cheaper than solar energy) - But none of the statements made in the argument point us towards the dropping price of oil.
C) Now, it becomes easier to choose C) via elimination. If one has eliminated A) the choice is obviously C). But if we think about it - The argument says the threshold economic viability between solar power and oil remains unchanged. This can mean the amount of energy generated by solar energy isn't useful enough even though cost of production/conversion has reduced. On the other hand, the word 'New' is a clue. 'New oil fired plants' which means these new ones are more efficient for some reason, one of them can be Technological Advances. So if one is confused about A). Thinking about C) this way can bring us closer to the answer as well
So, Answer is C.
I hope this helps.