平面向量 $\stackrel{\rightharpoonup }{a}=(1,2),\stackrel{\rightharpoonup }{b}(4,2),\stackrel{\rightharpoonup }{c}=m\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}(m\in R)$，且 $\stackrel{\rightharpoonup }{c}$与 $\stackrel{\rightharpoonup }{a}$的夹角等于 $\stackrel{\rightharpoonup }{c}$与 $\stackrel{\rightharpoonup }{b}$的夹角，则 $m=$（  ）

A．

-2

B．

-1

C．

1

D．

2