Astronauts require preserved food for extended flights.
Freeze-drying freeze drying is a method of preserving materials.
Freeze drying food keeps it viable for extended flights. .
Which of the following keeps food viable for extended flights? 
Options:
a: evaporation
b: microwaving
c: boiling
d: freeze drying
e: layers of fat
f: deep frying
g: mechanical
h: Dehydration