(20 12? Qinzhou) As shown in the figure, curves X, Y and Z respectively represent the digestibility of three organic substances in food in the digestive tract, which can represent the consumption of s

(20 12? Qinzhou) As shown in the figure, curves X, Y and Z respectively represent the digestibility of three organic substances in food in the digestive tract, which can represent the consumption of steamed bread. Starch is digested in the mouth, protein in the stomach and fat in the small intestine.

As can be seen from the figure, the substance X is digested first, then the substance Y is digested, and finally the substance Z is digested. It can be judged that X is starch, Y stands for protein and Z stands for fat.

In the figure, X, Y and Z can all be digested in D digestive tract, so it can be judged that D stands for small intestine; X begins to digest in a, then a stands for oral cavity; Y begins to digest in C, then C stands for stomach.

So choose: a