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