已知 $\cos \alpha=\frac{2 \sqrt{5}}{5}, \sin \beta=\frac{\sqrt{10}}{10}$, 且 $\alpha \in\left(0, \frac{\pi}{2}\right), \beta \in\left(0, \frac{\pi}{2}\right)$, 则 $\alpha+\beta$ 的值是
A. $\frac{3 \pi}{4}$
B. $\frac{\pi}{4}$
C. $\frac{7 \pi}{4}$
D. $\frac{5 \pi}{4}$