Free SAT Practice Question

Food scientist Renata Volkov studies fermented foods that people have prepared for centuries without any knowledge of the microbes at work in them. Cooks refined these methods by taste and habit alone, long before anyone could see a single bacterium under a lens. ______ the deep, savory flavor of a traditional aged soybean paste comes almost entirely from microbes that generations of cooks fed and encouraged without ever realizing the organisms were there.

Which choice completes the text with the most logical transition?