Scenario II A Psychologist Was Interested in the Effects of Sleep Deprivation

Scenario II
A psychologist was interested in the effects of sleep deprivation on performance on a vigilance task. Forty college students served as participants and were placed in either the sleep-deprivation group or the control group by coin flip. Both groups spent the night before the study in a sleep laboratory, but only the control group was allowed to sleep. By the next morning, the sleep-deprived group had been awake for the last 24 hours. At this time, both groups were provided a nutritious breakfast and, shortly thereafter, testing began in sound-attenuating cubicles, each equipped with a computer. The vigilance task consisted of monitoring the computer screen. Participants were instructed that red dots represented allied space crafts and green dots represented enemy space crafts. Throughout the 2-hr hour long task, red dots moved across the screen in irregular patterns. Occasionally and unpredictably, a green dot would quickly move across the screen in a haphazard pattern. When a green dot appeared, the task of the participant was to move a stylus over the green dot and press a button, "destroying" the enemy craft.
The results of this fictitious experiment are shown in Figure 2.2.
Figure 2.2.
-(Scenario II) The independent variable was:

A) the length of the vigilance task.
B) the instructions given.
C) degree of vigilance.
D) sleep status.