A 26-Year-Old Biologist Makes a Trip to Study Alien Life-Forms

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 distant planet takes 21.5 years according to the clock on earth. The biologist's son is 3 years old when he leaves. Assume that the planet is not moving with respect to Earth, and the speed is the same for both the outbound and inbound parts of the journey. How fast is the spaceship traveling?

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