An airline flight of 5 hours in the middle latitudes at an altitude of 12 km may result in an exposure of _____  µSv.
25
50
250
100
Directly picked from White and Pharoah:
"An airline flight of 5 hours in the middle latitudes at an altitude of 12 km may result in an exposure of about 25 µSv".
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition