题目

I remember the first time I got on a horse. When I was a little boy aged two, my mom agreed to let me take a short ride and that was it! From then on, I drove my parents crazy begging for a horse.When I was four, I had mutism, in which children stop speaking in certain social situations. I went days, weeks, months without a sound at school. At most, I might quietly whisper to a friend. I suffered silently through school until I was ten when a psychologist (心理学家) had an idea. He asked me what I wanted more than anything else in the world. He explained I was going to be given a chance to已知反应A+2B═C+2D，8gA与足量的B恰好完全反应生成22gC和18gD，D的相对分子质量为18，则A的相对分子质量为（　　）A．8B．18C．22D．16