3. Given that the dihedral angle $\alpha-l-\beta$ is $30^{\circ}$. Then the eccentricity of the ellipse obtained by the orthogonal projection of a circle on plane $\alpha$ onto plane $\beta$ is ( ).
(A) $\frac{1}{3}$
(B) $\frac{1}{2}$
(C) $\frac{\sqrt{3}}{3}$
(D) $\frac{\sqrt{3}}{2}$
3. B.

From $b=a \cos 30^{\circ}=\frac{\sqrt{3}}{2} a \Rightarrow c=\frac{1}{2} a$ $\Rightarrow e=\frac{c}{a}=\frac{1}{2}$.