Because of steep increases in the average price per box of cereal over

I approach the evaluate questions by following a simple technique - determine whether the answer choice is going to strenghthen or weaken the conclusion in concrete terms - YES or NO.
If the answer choice is either of the two that is going to help the argument and give us an answer.
Applying the technique above to this question:

Conclusion - Reduction in wholesale prices will have an effect on increase in sales due to more spending by the consumers on cereals.
Some pre thinking steps:
1. What if the consumer does not get the benefit of the wholesale prices and still pays the price as he was paying before.
Answer to this would help us evaluate the argument.
2. What if the consumer does not buy wholesale cereals and only buys from retail - then too the argument would give us our answer.

Applying the technique here:

A. Whether the high marketing expenses of the highly competitive cereal market led to the increase in cereal prices
- no effect

B. Whether cereal manufacturers use marketing techniques that encourage brand loyalty among consumers
- no effect

C. Whether the variety of cereals available on the market has significantly increased over the last 10 years
- no effect

D. Whether the prices that supermarkets charge for these cereals will reflect the lower prices the supermarkets will be paying the manufacturers
- Bingo , if the benefit is not getting passed over to the end user the prices will not reduce

E. Whether the sales of certain types of cereal have declined disproportionately over the last 10 years[/quote]
- no effect