Alex and Alexa are twins. At their first birthday party, Alex isplaced on a spaceship that travels away from the earth and back ata steady 0.85c. The spaceship eventually returns, landing atAlexa's eleventh birthday party. When Alex emerges from theship, it is discovered that:

A. He is still a year old

B. He is 6 years old

C. He is also 11 years old

D. He is 21 years old