A Spaceship Traveling at Constant Velocity Passes by Earth and Later

Multiple Choice

A spaceship traveling at constant velocity passes by Earth and later passes by Mars. In which frame of reference is the amount of time separating these two events the proper time?

A) any frame of reference, inertial or not
B) the spaceship frame of reference
C) the Mars frame of reference
D) any inertial frame of reference
E) the Earth frame of reference