参考答案

正确答案：B
解析：根据题干关键词experiment，plain-smelling room和选项中的阿拉伯数字定位到原文第五段：... each study participant received$12, making it seem as though the senders had entrusted them with the full $4 they had been given...On average, those in the plain-smelling room returned $2.5 to the sender, pocketing the lion's share of the money.可知实验中发放者给了每个人12美元，但最终实验对象平均送回2.81美元，而把剩余部分自己留下了，即留下了12-2.5=9.5美元。