In the Experiment in Which College Students Breathed Air with Increased

In the experiment in which college students breathed air with increased amounts of carbon dioxide through a mask, the students found it much LESS stressful if they

A) were surprised by the trials that contained the increased amount of carbon dioxide.
B) were told which trial would contain an increased amount of carbon dioxide.
C) did not have to control the amount of carbon dioxide entering the mask.
D) experienced repeated trials of the "surprise whiffs" of carbon dioxide in increased amounts.