How did NASA help to distinguish genetic factors from environmental ones when studying the effects of living space?

A. By observing changes in retired astronauts over a period of ten years.
B. By introducing animals and other organisms that could be studied in space.
C. By running experiments on astronauts the were in space for over a year.
D. By analyzing changes in one twin on Earth and the other in space.

D. By analyzing changes in one twin on Earth and the other in space.