If variables x and y have a perfect positive correlation (r = 1.0), and an observation of x is one standard deviation of x below the mean of x, what should the value of y be?

a. One standard dedication of y below the mean of x?
b. one standard deviation of x below the mean of x?
c. one standard deviation of x below the mean of y?
d. One standard deviation of y below the mean of y?