I think it is D.
Actually I looked at B and E also. I pondered over E for long!! But, only D makes the statement that asserts the way the identical quality is maintained.
Although many brands of gasoline are sold on Haibei island, gasoline companies there get all of the refined gasoline they sell from Haibei seaport's only storage tank, which is always refilled with the same quality of gasoline. Therefore, the brands of gasoline for sale on Haibei may be different in name and price, but there are identical in quality.
The conclusion drawn above depends on which of the following assumptions?
a. consumers are usually unaware of variations in the quality of the gasoline they buy unless those variations are announced by the gasoline companies.
b. When tankers make gasoline delivery at Haibei's seaport, the storage tank an Haibei always receives the same quality of gasoline as that in the preceding delivery.
c. There is a wide variation in the price at which the deferent brands of gasoline on Haibei are sold.
d. if any gasoline company on Haibei alters the quality of its gasoline before sale, the other gasoline companies also use method before sale that result in the same change in the quality of their gasoline.
e. The gasoline storage tank on Haibei's large enough to meet the needs of all of Haibei's different gasoline companies.
Thnx & Rgds,