From 1978 to 1988, beverage containers accounted for a steadily decreasing % of the total weight of household garbage in the United States. The increasingly widespread recycling of aluminum and glass was responsible for most of this decline. However, although aluminum recycling was more widely practiced than glass recycling, it was found that the weight of glass bottles in household garbage declined by a greater % than the weight of the aluminum cans.

Which of the following, if true of the United States in the period 1978 to 1988, most helps to account for the finding?

(a) Glass bottles are significantly heavier than aluminum cans of comparable size.
This shouldn't matter. We are talking about the % decline over a period of time, not the overall weight.

(b) Recycled aluminum cans were almost all beverage containers, but a significant fraction of the recycled glass bottles had contained products other than beverages.
This also shouldn't matter. Who cares what is inside the recycled glass bottles--the bottles are in the trash, so the contents of what was in them should be emptied or somewhere else. The conclusion is about the bottles in the trash, not the bottles before they enter the trash. Even if it were true, what's inside the glass bottles could be heavier or lighter than the aluminum cans.

(c) Manufacturers replaced many glass bottles, but few aluminum cans, with plastic containers.
This is our answer. If manufacturers replaced the glass bottles with plastic, as a %, the weight of the glass bottles compared to everything else would go down. Since this says aluminum cans were replaced fewer than glass bottles, the conclusion makes logical sense.

(d) The total weight of glass bottles purchased by households increased at a slightly faster rate than the total weight of aluminum cans.
If this were true, it would mean the conclusion of the stimulus should be exactly the opposite.

(e) In many areas, glass bottles had to be sorted by color of the glass before being recycled, whereas the aluminum cans required no sorting.
This has nothing to do with the weight.