单项选择题
A.上清中有少量游离抗体B.上清中有少量游离抗原C.上清中几乎无游离抗原或抗体存在D.出现沉淀物是肉眼不可见的
A.they did not queue for the busB.they were foreignersC.the people were unfriendlyD.there were not any reasons