A pair of six-sided fair dice are labeled so that one die has only even numbers (two each of 2,4, and 6), and the other die has only odd numbers (two each of 1,3 , and 5). The pair of dice is rolled. What is the probability that the sum of the numbers on the tops of the two dice is 7 ?
Answer Choices
A. \dfrac{1}{6}
B. \dfrac{1}{5}
C. \dfrac{1}{4}
D. \dfrac{1}{3}
E. \dfrac{1}{2}