题目

When I was about twelve, I was going to a restaurant for dinner with my family. It was winter, and on that    21    night, the wind was really blowing. As my mom and I headed towards the    22    from our car, a girl about my age and her mother came up to us. They asked if we had any    23    change. My mom immediately asked where they kept their things. They pointed to an old car in the parking lot across the street. The girl said there were six of them    24    in that car. After    25    them a few dollars, my mom said she had    26    to do and 研究发现两种现象：①动物体内的B细胞受到抗原刺激后，在物质甲的作用下，可增殖、分化为效应B细胞；②给动物注射从某种细菌获得的物质乙后，此动物对这种细菌具有了免疫能力。则这两种物质中(    ) A．甲是抗体，乙是抗原         B．甲是抗体，乙是抗原 C．甲是白细胞介素-2，乙是抗体   D．甲是白细胞介素-2，乙是抗原