若 $\stackrel{\rightharpoonup }{a_1},\stackrel{\rightharpoonup }{a_2},\stackrel{\rightharpoonup }{a_3}$均为单位向量，则 $\stackrel{\rightharpoonup }{a_1}=(\frac{\sqrt{3}}{3},\frac{\sqrt{6}}{3})$是 $\stackrel{\rightharpoonup }{a_1}+\stackrel{\rightharpoonup }{a_2}+\stackrel{\rightharpoonup }{a_3}=(\sqrt{3},\sqrt{6})$的

A．	充分不必要条件．	B．	必要不充分条件．
C．	充要条件．	D．	既不充分也不必要条件