Some Friends Are Trying to Make Wine in Their Basement

Some friends are trying to make wine in their basement. They've added yeast to a sweet grape juice mixture and have allowed the yeast to grow. After several days they find that sugar levels in the grape juice have dropped, but there's no alcohol in the mixture. The most likely explanation is that the

A) mixture needs more sugar because yeasts need a lot of energy before they can begin to produce alcohol.
B) mixture needs less oxygen because yeasts only produce alcohol in the absence of oxygen.
C) mixture needs more oxygen because yeasts need oxygen to break down sugar and get enough energy to produce alcohol.
D) yeast used the alcohol as a carbon source.