hi, ywilfred
Now I've been convinced that D is better than E.
Let's look the passage carefully, and I will divide the passage into several parts.
1, the cost of producing electric power at photovoltaic power plants is one-tenth of what it was 20 years ago. Let's set
(1/10x, now) and
(x, 20 years before).
2, the corresponding cost for traditional plants has increased
3, conclusion: photovoltaic power plants offer a less expensive approach than do traditional power plants.
Caution: there is a big gap between 1,2 and 3, we need to compare the
cost of photovoltaic plants with
the cost of traditional plants so as to get the conclusion.
in Choice 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.
We don't need to assume any further decreasing of photovoltaic plants. As long as the cost of photovoltaic plants is less than the cost of traditional plants now or 20 years before, we can attain the conclusion.
Now read D again.
(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.
This exactly supplements the big gap.
That is to say, the cost of traditional plants is 20x in my example.
Moreover, becaus of technology advance and the corresponding increasing cost for traditional plants, we can get the conclusion: photovoltaic power plants offer a less expensive approach than do traditional power plants.
Hope this helps.
Point noted. Thanks
I do not know how i got my math for this question all wrong.
close
I do not know how i got my math for this question all wrong. 
