A lossless antenna has directional gain $$ G(\theta,\psi)$$, then $$\int_{\psi = -\frac{\pi}{2}}^{\psi = \frac{\pi}{2}}$$ $$\int_{\theta = -\frac{\pi}{2}}^{\theta = \frac{\pi}{2}} G(\theta, \psi)d \theta d \psi$$ is:

A

$$4\pi$$

B

$$2\pi$$

C

$$ \leq 4\pi$$

D

$$ \leq 2\pi$$