Point B - Zero Rate
In this particular example, adding a zero treatment ("B") would not make the difference any clearer, although it is certainly helpful where there is a difference in the maximum yield from each treatment. A common marketing ploy is to include the Normal fertilizer at the usual rate ("A"), with the Enhanced product at a reduced rate ("B"), and conclude that the Enhanced material is more efficient because it gave similar yield for less input. While this is correct in this example, this conclusion is accidental rather than firm proof because the same results would be seen if the rates for both products were on the yield plateau.

Point C - Expected Response Difference
The valid comparison that would show the difference between the two products would include both products at a rate where there is a difference in response, as at point "C". Since we don't know prior to the trial exactly where this point will be, the most reliable design includes multiple rates of both products so that a yield response curve can be drawn for each.