Two astronauts travel on spaceships that are both moving with constant velocities, but at different speeds. On which of the following quantities will the astronauts agree? In each case, give a reason for your answer. 

(a) The length of an object (e.g., a third spaceship) 

(b) The time interval between two events 

(c) The speed of light in a vacuum 

(d) The speed of a moving object (e.g., a third spaceship) 

(e) The relative speed of the observers