There is a list of seven numbers. The average of the first four numbers is 5 , and the average of the last four numbers is 8 . If the average of all seven numbers is 6 \dfrac{4}{7}, then the number common to both sets of four numbers is
Answer Choices
A. 5 \dfrac{3}{7}
B. 6
C. 6 \dfrac{4}{7}
D. 7
E. 7 \dfrac{3}{7}