问题详情

How did the researchers test their volunteers？
A．By giving everyone nasal drops containing either a cold virus or a particular flu virus．
B．By giving everyone a medicine that help lessen the probability of catching cold．
C．By giving everyone an injection boosting immune function．
D．By investigating everyone’s characteristics，interests and hobbies． 请帮忙给出正确答案和分析，谢谢！