设 a⇀,b⇀ 是两个非零的平面向量，下列说法正确的是（    ）

A：$\left|\stackrel{\rightharpoonup }{a}•\stackrel{\rightharpoonup }{b}|=|\stackrel{\rightharpoonup }{a}\right|\left|\stackrel{\rightharpoonup }{b}\right|$

B：向量 $\stackrel{\rightharpoonup }{a}$ 在向量 $\stackrel{\rightharpoonup }{b}$ 方向上的投影为 $\left|\stackrel{\rightharpoonup }{a}\right|\text{cos}\langle \stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}\rangle$

C：$\left|\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}|=|\stackrel{\rightharpoonup }{a}|-|\stackrel{\rightharpoonup }{b}\right|$ ，则 $\stackrel{\rightharpoonup }{a}\perp \stackrel{\rightharpoonup }{b}$

D：若 $\stackrel{\rightharpoonup }{a}//\stackrel{\rightharpoonup }{b}$ ，则有 $\left|\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}|=|\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|$