题目

On a British Airways flight from Johannesburg, a middle-aged ,rich white South American lady found herself sitting next to a black man. She called the cabin stewardess(机舱乘务员)over to complain about her seating. “What seems to be the problem, Madam?” asked the stewardess. “Can’t you see?” she said. “You’ve sat me next to a black man. I can’t possibly sit next to this kind of human. Find me another seat!” “Please calm down, Madam,” the stewardess replied. “The flight is very full today, but I’ll tell you what I’ll do---I Will go and check to see if we 17．用同位素标记法追踪元素及物质的去向是生物学研究的重要手段之一．下列结果正确的是（　　）A．用18O分别标记H2O和CO2证实了CO2是光合作用的原料B．用35S标记噬菌体侵染大肠杆菌的实验中，搅拌不充分会导致沉淀物存在少量放射性C．用含有放射性碘的饲料饲喂成年狗，可在血液中的促甲状腺激素中检测到放射性D．用含3H标记尿嘧啶的营养液培养洋葱根尖，可以在细胞核和线粒体处检测到较强的放射性，而在核糖体处则检测不到