A, the small intestine is the main place to absorb nutrition. After blood flows through the small intestine, nutrients will increase significantly; Blood circulation through the small intestine is a part of systemic circulation. Therefore, after blood flows through the small intestine, oxygen will decrease and carbon dioxide will increase, but oxygen will not decrease to zero. So, A is wrong.

B, from the immune mechanism analysis of the interaction between antigen and antibody, measles vaccine is equivalent to antigen. After being inoculated into the human body, it will stimulate lymphocytes in the human body to produce antibodies, thus playing an immune role against measles virus. After the antibody destroys the antigen, the antibody still exists, so the antigen content changes from variable to less; The content of antibody changed from less to more, so B is correct.

C, yeast fermentation requires an anaerobic environment, and the container is sealed. Due to the propagation of yeast, the limited oxygen in the sealed container will soon be consumed, so the rapidly reduced substance 1 is oxygen; In the initial process of growth and reproduction, carbon dioxide will be produced, and after sufficient reproduction, 2 alcohols and carbon dioxide will be produced by fermentation, so it is wrong to have the carbon dioxide content in the image much higher than 2 alcohols at first. So, C is wrong.

D Frog belongs to a kind of temperature-changing animal, and its body temperature changes with the change of environment. So in winter, frogs hibernate and their metabolism is very slow, so the oxygen consumption is very low, but it is not zero. So, d is wrong.

So choose: B.