The covariance of the random variables X and Y if each of them takes only 3 values (1,2,3} with the following probabilities : 1 2 3 10 1/6 1/6 y 2 1/6 0 0 1/6 3 1/6 1/6 Select one: a. none of these O b. 22/6 O c. 1/3 O d. -1/3