$A$ family consists of grandparents,parents,and three grandchildren. The average age of the grandparents is $67 \text{ years}$,that of the parents is $35 \text{ years}$,and that of the grandchildren is $6 \text{ years}$. What is the average age of the family? (in $\text{years}$)

• A
  $28 \frac{4}{7}$
• B
  $31 \frac{5}{7}$
• C
  $32 \frac{1}{7}$
• D
  $29 \frac{4}{7}$