Crystal Is Studying the Effects of Sleep Deprivation on Exam

Crystal is studying the effects of sleep deprivation on exam performance. In her experiment, she randomly assigned fifty men and fifty women into two different groups. The first group could sleep eight hours a night for one week and the second group could sleep for only four hours a night for one week. All participants were given a book to study from at the start of the experiment and
Told they would be tested on the material at the end of the week. The group that had slept eight hours a night scored significantly higher on the exam than the group that had slept four hours a night. What was the independent variable for this study?

A) sex of participant
B) hours of sleep
C) the exam
D) the topic of the exam
E) random assignment