Achild on a playground swing is swinging back and forth (one complete oscillation) once every four seconds, as seen by her father standing next to the swing. at the same time, a spaceship is hurtling by at a speed close to the speed of light. according to special relativity (and ignoring the doppler effect for this question), her mother on the spaceship finds that the time for one full swing is
a) less than four seconds when the spaceship is approaching the swing and longer than four seconds when it is moving away.
b) less than four seconds.
c) equal to four seconds.
d) longer than four seconds.