A、When they see the steam and ice in the tank. B、When they see the actors filling the containers. C、When they see the actors put

Just watching another person shiver can cause our own temperature to drop, scientists have found. Volunteers who viewed videos of actors plunging their hands into ice-cold water, experienced a simultaneous drop in the temperature of their own hands. Neuroscientist Neil Harrison, from the University of Sussex said, "We believe that this bodily response helps us understand how they are feeling. Humans are profoundly social creatures and much of humans’ success results from our ability to work together in complex communities—this would be hard to do if we were not able to rapidly share feelings with each other and predict one another’s thoughts, feelings and motivations. "
    For the study, researchers asked 36 participants to watch eight videos that showed actors with one of their hands in visibly warm or cold water. In the four "warm" videos, the first 40 seconds showed the actor gradually adding hot water from a steaming kettle into the container, checking the temperature of the water every few seconds. The actor did the same in the "cold" videos, but instead, filled the container with a bag of ice. Four control videos with the actors’ hands in front of a tank of room temperature water were also shown. The researchers monitored the participants’ hand temperature while they watched the videos. They found that when they viewed the actors putting their hands in the iced water, their temperatures dropped by a small, but statistically significant, amount: 0.2℃ in their left hands, and 0.05℃ in their right ones. There was no significant change in their hand temperature when they watched the control videos and the warm water videos.
16. What does neuroscientist Neil Harrison say about humans?
17. In what kind of water are the actors’ hands put in the control videos?
18. When does the participants’ hand temperature change?