C. We have no data to authenticate other choices.

My answer is D.

Premise:
Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances, the cost of producing electric power at photovoltaic power plants, allowing for both construction and operating costs, is one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants, which burn fossil fuels, has increased.
Conclusion:
Thus, photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.
Assumption:
The cost of photovolaic power was less then cost of power of traditional plants.
Now, looking for similar idea.

The conclusion of the argument is properly drawn if which one of the following is assumed?
(A) The cost of producing electric power at traditional plants has increased over the past 20 years.
Nope.
(B) Twenty years ago, traditional power plants were producing 10 times more electric power than were photovoltaic plants.
Nope.
(C) None of the recent technological advances in producing electric power at photovoltaic plants can be applied to producing power at traditional plants.
Nope.
(D) Twenty years ago, the cost of producing electric power at photovoltaic plants was less than 20 times the cost of producing power at traditional plants.
BINGO! The same idea

(E) The cost of producing electric power at photovoltaic plants is expected to decrease further, while the cost of producing power at traditional plants is not expected to decrease.
Nope.
Now try denial test. 20 year ago the cost of producing electric power at photovoltaic plants was MORE then 20 times the cost of producing power at traditional plants. Our conclusion falls apart. THUS, this is necessary assumption!

I agree with (C).

As per D the power from solar plants is <2 times that produced from traditional plants.
This does not mean however that cost of power from solar plants is lower than cost of power from traditional plants

http://www.gmatclub.com/phpbb/viewtopic.php?t=7086