The average age of $8$ members of a group is $40 \text{ years}$. The age of $1$ member is $55 \text{ years}$. After his retirement,a new member of age $39 \text{ years}$ joins the group. What is the effect of this replacement on the average age of the group?
- A$2 \text{ years decrease}$
- B$0 \text{ years}$
- C$3 \text{ years decrease}$
- D$5 \text{ years decrease}$
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