若向量$\stackrel{\rightharpoonup }{a}=\left(1,2\right),\stackrel{\rightharpoonup }{b}=\left(1,-1\right)$，则$2\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}与\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}$的夹角等于（）

A．

- $\frac{\pi }{4}$

B．

$\frac{\pi }{6}$

C．

$\frac{\pi }{4}$

D．

$\frac{3\pi }{4}$