3. If $D$ is a point on side $B C$ of $\triangle A B C$, and the centroids of $\triangle A B D$ and $\triangle A C D$ are $E, F$ respectively, then $S_{\triangle A E F}=S_{\triangle A B C}$ equals ).
(A) $4 \div 9$
(B) $1: 3$
(C) $2 \div 9$
(D) $1: 9$
3. $\mathrm{C}$ (Hint: Extend $A E, A F$ to intersect $B C$ at $M, N$. Then $M, N$ are the midpoints of $B D, D C$ respectively.)