$△ABC$中， $AB$边的高为 $CD$，若 $\stackrel{\to }{CB}=\stackrel{\rightharpoonup }{a},\stackrel{\to }{CA}=\stackrel{\rightharpoonup }{b}$， $\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}=0,\left|\stackrel{\rightharpoonup }{a}\right|=1,\left|\stackrel{\rightharpoonup }{b}\right|=2$，则 $\stackrel{\to }{AD}$=

A．	$\frac{1}{3}\stackrel{\rightharpoonup }{a}-\frac{1}{3}\stackrel{\rightharpoonup }{b}$	B．	$\frac{2}{3}\stackrel{\rightharpoonup }{a}-\frac{2}{3}\stackrel{\rightharpoonup }{b}$
C．	$\frac{3}{5}\stackrel{\rightharpoonup }{a}-\frac{3}{5}\stackrel{\rightharpoonup }{b}$	D．	$\frac{4}{5}\stackrel{\rightharpoonup }{a}-\frac{4}{5}\stackrel{\rightharpoonup }{b}$