7. As shown in the figure, the angle between the oblique line segment AB and plane $\alpha$ is $60 ^ { \circ }$ , B is the foot of the oblique line, and the moving point P on plane $\alpha$ satisfies $\angle \mathrm { PAB } = 30 ^ { \circ }$ . Then the locus of point P is\\
A. a line\\
B. a parabola\\
C. an ellipse\\
D. one branch of a hyperbola