Time Dilation: a Set of Twins, Andrea and Courtney, Are

Multiple Choice

Time dilation: A set of twins, Andrea and Courtney, are initially 10 years old. While Courtney remains on Earth, Andrea rides on a spaceship which travels away from Earth at a speed of 0.60c for five years (as measured by Courtney) , then immediately turns around and comes back at 0.60c. When Andrea returns, Courtney is 20 years old. How old is Andrea upon her return?

A) 10 y
B) 12 y
C) 15 y
D) 18 y
E) 20 y