9.平面直角坐标系xOy中,已知A(-1,0)、B(3,0)、C(0,-1)三点,D(1,m)是一个动点,当△ACD的周长最小时,△ABD的面积为( )
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| A. | $\frac{1}{3}$ | B. | $\frac{2}{3}$ | C. | $\frac{4}{3}$ | D. | $\frac{8}{3}$ |