![](https://cos-cdn.shuashuati.com/pipixue-web/2020-1231-2005-12/ti_inject-812ce.png)
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.