[单选题]
A spaceship is moving at a constant velocity relative to a station. The spacetime diagram shows the worldline of the spaceship in the frame of reference of the station.
Which of the following gives the speed of the spaceship in terms of $c$?
A. the shaded area
B. $\frac{1}{The\,shaded\,area}$
C. $tan θ$
D. $\frac{1}{tanθ}$
