I think the developments the passage is talking about are:
1. Discovery of leavened bread
2. Introduction of Wheat Variety
and we need to show how they are casually related.
Now, I'm not much of a baker so I'm only guessing that even to bake unleavened bread, you would have to bake the flour, ruling B out.
also, the argument says that the Egyptians discovered the Leavened bread when they switched to the Wheat variety. So, we would need to find the reason which would identify, why did the Egyptians switch to the Wheat variety.
and E provides us with that.
Of course, the underlying assumption is that even to make unleavened bread, you'll need to bake it.
