One of two 25-year-old identical twins begins a trip on a spaceship traveling at 0.8 c while her twin remains on Earth. The twin on Earth tracks her twin's trip for 10 years. After ten years, what age is the twin on the spaceship according to the twin on Earth? a. 41.7 years old.
b. 35 years old.
c. 31 years old.
d. 37.5 years old.