A set of 10 cards consists of 5 red cards and 5 black cards. The cards are shuffled thoroughly and I choose one at random, observe its color, and replace it in the set. The cards are thoroughly reshuffled, and I again choose a card at random, observe its color, and replace it in the set. This is done a total of four times. Let X be the number of red cards observed in these four trials. The variance of X is
A. 4.
B. 2.
C. 1.
D. 0.5.