Use the Following Scenario for the Next Three Questions

Multiple Choice

Use the following scenario for the next three questions.
A 26-year-old biologist makes a trip to study alien life forms in a distant planet 10 light-years away.The round trip including a stay of 1 year in the distance planet takes 21.5 years according to the clock on earth.The biologist's son is 3-year-old when he left.Assume that the planet is not moving with respect to Earth,and the speed is the same for both outbound and inbound part of the journey.
-How fast is the spaceship traveling?

A) 0.375c
B) 0.5c
C) 0.67c
D) 0.845c
E) 0.976c