已知向量 a⇀=(2,0),b⇀=(−1,−1), 则下列结论正确的是（    ）

A：$\stackrel{\rightharpoonup }{a}\cdot \stackrel{\rightharpoonup }{b}\text{=}2$

B：$\stackrel{\rightharpoonup }{a}//\stackrel{\rightharpoonup }{b}$

C：$\stackrel{\rightharpoonup }{b}\perp (\stackrel{\rightharpoonup }{a}\text{+}\stackrel{\rightharpoonup }{b}\text{)}$

D：$\left|\stackrel{\rightharpoonup }{a}\right|\text{=}\left|\stackrel{\rightharpoonup }{b}\right|$