如图所示\( ,\)点D是△ABC的边AB的中点\( ,\)则向量\( \overrightarrow{CD}\)等于( )
A. \( -\overrightarrow{BC}-\frac{1}{2}\overrightarrow{BA}\)
B. \( \overrightarrow{BC}+\frac{1}{2}\overrightarrow{BA}\) C. \( -\overrightarrow{BC}+\frac{1}{2}\overrightarrow{BA}\) D. \( \overrightarrow{BC}-\frac{1}{2}\overrightarrow{BA}\)
