If $$\tan x = \frac{m}{n}$$ and $$a^{\circ} \leq x \leq 90^{\circ}$$, then the value of $$(\sin x + \cos x)$$ is:

A

$$\sqrt{m^2 -n^2}$$

B

$$\frac{1}{\sqrt{m^2 - n^2}}$$

C

$$\frac{m + n}{\sqrt{m^2 + n^2}}$$

D

$$\frac{1}{\sqrt{m^2 + n^2}}$$