Yeasts capable of leavening bread are widespread, and in the many centuries during which the ancient Egyptians made only unleavened bread, such yeasts must frequently have been mixed into bread doughs accidentally. The Egyptians, however, did not discover leavened bread until about 3000 B. C. That discovery roughly coincided with the introduction of a wheat variety that was preferable to previous varieties because its edible kernel could be removed from the husk without first toasting the grain. Which of the following, if true, provides the strongest evidence that the two developments were causally related? ['Even after the ancient Egyptians discovered leavened bread and the techniques for reliably producing it were well known, unleavened bread continued to be widely consumed.', 'Only when the Egyptians stopped the practice of toasting grain were their stone-lined grain-toasting pits available for baking bread.', 'The new variety of wheat, which had a more delicate flavor because it was not toasted, was reserved for the consumption of high officials when it first began to be grown.', 'Heating a wheat kernel destroys its gluten, a protein that must be present in order for yeast to leaven bread dough.']