$17 \cdot 102$ In the figure, $D$ is the midpoint of side $BC$ of $\triangle ABC$, $E$ is an internal point on side $AC$, $AC=3CE$, $BE$ and $AD$ intersect at $G$. Then $AG: GD$ is
(A) 2 .
(B) 3 .
(C) 3 or 4 .
(D) 4 .
$\begin{array}{l}\text { [Solution] Draw } D F / / B E \text { intersecting } A C \text { at } F \text {. } \\ \text { Since } D \text { is the midpoint of } B C \text {, then } F \text { is the midpoint of } E C \text {. } \\ \text { Given } A C=3 C E \text {, we have } A E=2 C E=4 E F \text {, } \\ \text { and } D F / / B E, \\ \text { thus } A G: G D=A E: E F=4: 1=4 . \\ \text { Therefore, the answer is }(D) .\end{array}$