The accompanying data set contains two variables: x1 and x2. Observation x1 x2 1 0.46 4 2 0.41 1 3 0.96 4 4 0.48 2 5 0.79 3 6 0.70 2 7 0.14 3 8 3 9 0.49 3 10 0.84 2 11 0.90 3 12 0.04 1 13 0.19 3 14 0.06 2 15 0.23 2 How many observations have x2 values equal to 2? a. 3 b. 5 c. 15 d. 0