In order to save money, some of Company X's manufacturing plants converted from oil fuel to natural gas last year, when the cost of oil was more than the cost of natural gas. Because of a sudden, unexpected shortage, however, natural gas now costs more than oil, the price of which has fallen steeply over the past year. The cost of conversion back to oil would more than negate any cost savings in fuel. So Company X's fuel costs this year will be significantly higher than they were last year.
Which of the following is an assumption on which the argument above depends?
(A) Company X does not have money set aside for the increased costs of fuel.
It's pretty obvious that this answer choice has nothing to do with the conclusion, which states that Company X's fuel costs this year will be significantly higher than they were last year.
(B) The increase in the cost of fuel cannot be offset by reductions in other operating expenses.
The conclusion is very specific about rising fuel bill . This answer choice deals with "total costs". It is very possible that fuel costs are the highest ever and the total costs are the lowest ever. Unlikely,but definitely possible
(C) The price of natural gas will never again fall below that of oil.
This is a very tricky answer choice. The conclusion talks about [b]total fuel costs for this year.
Assume that the company closes its financial books on Dec 31st. It is very possible that fuel prices crash in Dec 30 to their all time lows. Despite this possibility, the fuel costs for the company can be higher than last year[/b]
(D) The cost of fuel needed by those of Company X's plants that converted to natural gas is not less than the cost of fuel needed by those plants still using oil.
A-Ha. Finally, we have an answer choice that addresses the real issue of "total fuel price". Just to be sure, let's negate this answer choice
The cost of fuel needed by those of Company X's plants that converted to natural gas is LESSthan the cost of fuel needed by those plants still using oil.
This shatters the conclusion. An assumption is an integral part of the argument. Negate the assumption and the conclusion falls apart like Romney in the presidential debate 
Total fuel costs Last year = Total Natural Gas Costs last year + Total Oil Costs last year
Total fuel costs this year = Total Natural Gas Costs this year + Total Oil Costs this year
Price of Natural Gas has significantly increased and price of Oil has gone down significantly.
If Total fuel costs this year needs to be significantly > Total fuel costs Last year, that implies
Total Natural Gas Costs this year >>> Total Oil Costs this year
Why ?
Because Total Oil Costs this year is much lesser than total oil cost last year.
(E) The price of oil will not experience a sudden and steep increase.
Do we really care about price of oil. Our comparison is the price of Natural Gas fuel this year Vs the price of oil LAST YEAR. This is like my neigbour winning a lottery. No material impact on my finances