The following data were obtained from a survey of college students. The variable X represents the number of non-assigned books read during the past six months.
x 0 1 2 3 4 5
P (X=x) 0.20 0.25 0.20 0.15 0.10 0.10
What is the standard deviation of X?
A. 1.58
B. 2.0
C. 1.24
D. 3.24