If we have oil:vinegar:water as 3x:2x:1x, then we have 6x total parts per cup.
Let's see where we are limited.
1/2 of these parts (3x/6x) are oil: therefore, half a cup of each cup of dressing is oil. The number of 1/2-cup units in 8 cups is of course 8 * 2 = 16 potential cups of dressing.
Now we need to see whether we are limited any lower by vinegar:
1/3 of these parts (2x/6x) are vinegar: therefore, one-third of each cup of dressing is vinegar. By the same logic, we have 7*3 = 21 potential cups of dressing. This limits higher, and we are bound by the lower number, 16.
Answer E.
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