已知平面向量 $\stackrel{\rightharpoonup }{a}=(1,2),\stackrel{\rightharpoonup }{b}=(-2,m)$，且 $\stackrel{\rightharpoonup }{a}//\stackrel{\rightharpoonup }{b}$，则 $2\stackrel{\rightharpoonup }{a}+3\stackrel{\rightharpoonup }{b}$＝（  ）

A．

(-5,-10)

B．

(-4,-8)

C．

(-3,-6)

D．

(-2,-4)