【单选题】A spacecraft built in the shape of a sphere moves past an observer on the Earth with a speed of 0.500c. What shape does the observer measure for the spacecraft as it goes by?
A.
a sphere
B.
a cigar shape, elongated along the direction of motion
C.
a round pillow shape, flattened along the direction of motion
D.
a conical shape, pointing in the direction of motion