6. The range of the function $y=\sqrt{x-4}+\sqrt{15-3 x}$ is ( ).
A. $[1,2]$
B. $[0,2]$
C. $[\sqrt{2}, 2]$
D. $[\sqrt{3}, 2]$
6. A.

Let $\sqrt{x-4}=\cos \theta$, then $\sqrt{5-x}=\sin \theta, \theta \in\left[0, \frac{\pi}{2}\right]$, so $y=\cos \theta+\sqrt{3} \sin \theta=$ $2 \sin \left(\theta+\frac{\pi}{6}\right)$. Therefore, $1 \leqslant y \leqslant 2$.