设向量$\mathit{a}\mathbf{,}\mathit{b}$满足$\left|\stackrel{\mathbf{\rightharpoonup }}{\mathbf{a}}\mathbf{+}\stackrel{\mathbf{\rightharpoonup }}{\mathbf{b}}\right|=\sqrt{10},\left|\stackrel{\mathbf{\rightharpoonup }}{\mathbf{a}}\mathbf{-}\stackrel{\mathbf{\rightharpoonup }}{\mathbf{b}}\right|=\sqrt{6}$，则$\stackrel{\mathbf{\rightharpoonup }}{\mathbf{a}\mathbf{·}}\stackrel{\mathbf{\rightharpoonup }}{\mathbf{b}}=$（）

A．

1

B．

2

C．

3

D．

5