Simultaneity: Astronaut Mark Uri Is Space-Traveling from Planet X to Planet

Multiple Choice

Simultaneity: Astronaut Mark Uri is space-traveling from planet X to planet Y at a speed of relative to the planets, which are at rest relative to each other. When he is precisely halfway between the planets, a distance of 1.0 light-hour from each one as measured in the planet frame, nuclear devices are detonated on both planets. The explosions are simultaneous in Mark's frame. What is the difference in the time of arrival of the flashes from the explosions as observed by Mark?

A) 0 min
B) 180 min
C) 90 min
D) 360 min
E) 113 min