第2418题：方向角与方向余弦

设空间直角坐标系中有个单位向量

$\mathbf{r_1}=(\cos \alpha_1, \cos \beta_1, \cos \gamma_1)$ ，$\mathbf{r_2}=(\cos \alpha_2, \cos \beta_2, \cos \gamma_2)$ ，

那么这两个向量夹角的余弦 $\cos \theta$ 为（  ）。 

A. $\cos \theta =$$\cos \alpha_1 \cos \alpha_2 + \cos \beta_1 \cos \beta_2+ \cos \gamma_1 \cos \gamma_2$

B. $\cos \theta =$ $\sin \alpha_1 \sin \alpha_2 + \sin \beta_1 \sin \beta_2+ \sin \gamma_1 \sin \gamma_2$

C. $\cos \theta =$ $\cos \alpha_1 \sin \alpha_2 + \cos \beta_1 \sin \beta_2+ \cos \gamma_1 \sin \gamma_2$

D. $\cos \theta =$ $\sin \alpha_1 \cos \alpha_2 + \sin \beta_1 \cos \beta_2+ \sin \gamma_1 \cos \gamma_2$