【单选题】

Two groups of people are compared in the third study. They are people______.

A、with less than5 hours of sleep a night and with 7 to 9 hours of sleep
B、who slept 4 hours a night and who slept 4 hours for 2 nights
C、with less than 4 hours of sleep a night and with 7 to 9 hours of sleep
D、with less than 5 hours of sleep a night and with 8 hours of sleep