A Spaceship Moving Past the Earth with a Speed of 0.800

Multiple Choice

A spaceship moving past the Earth with a speed of 0.800 c signals to the Earth with pulsed laser photons emitted at 10.0-second intervals according to the spaceship's clock. According to observers on Earth who see the flashes, the time interval they measure is

A) 13.4 s
B) 16.7 s
C) 12.5 s
D) 9.70 s
E) 6.03 s