Analysis of test questions: Saccharomyces cerevisiae is needed for wine-making. Under anaerobic conditions, yeast fermentation can decompose glucose into alcohol and carbon dioxide. When making liqueur, the number of yeasts added first is too small, so it is necessary for yeasts to grow and reproduce in large quantities first. Therefore, air is first introduced for a period of time, and then the air is isolated, with the aim of allowing yeasts to carry out aerobic breathing first, rapidly increasing the number of yeasts, and then carrying out anaerobic breathing after oxygen consumption to produce alcohol, so the answer to this question is C.

Test center: This question examines the role of fermentation technology in food production.

Comments: This question is novel. Pay attention to understanding and mastering the role of fermentation technology in food production, and answer flexibly.