From 1978 to 1988, beverage containers accounted for a steadily decreasing percentage of the total weight of household garbage in the United States. The increasingly widespread practice of recycling aluminum and glass was responsible for most of this decline. However, although aluminum recycling was more widely practiced in this period than glass recycling, it was found that the weight of glass bottles in household garbage declined by a greater percentage than the weight of 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? ['Manufacturers replaced many glass bottles, but few aluminum cans, with plastic containers.', 'Glass bottles are significantly heavier than aluminum cans of comparable size.', 'Recycled aluminum cans were almost all beverage containers, but a significant fraction of the recycled glass bottles had contained products other than beverages.', 'In many areas, glass bottles had to be sorted by color of the glass before being recycled, whereas aluminum cans required no sorting.']