A student noticed that when she gets less than 8 hours of sleep at night, she scores lower on her test grades. Can one conclude that this positive correlation shows a casual relationship?
A. Yes, because it is well known that less sleep lowers test scores.
B. Yes, because it is not a negative correlation.
C. No, because all students get less than 8 hours of sleep at night.
D. No, because it is possible that the student gets lower test grades because of another factor, such as not studying.