A Person Who Sits for Hours on a Long Airplane

A person who sits for hours on a long airplane flight may have problems with blood pooling below the knees.Why?

A) The valves in the leg veins don't work above a certain altitude.
B) Blood pressure drops on long airplane flights.
C) The respiratory pump is not helping blood return to the heart.
D) The capillaries expand at high altitudes.
E) The skeletal muscle pump below the knees is not helping blood return to the heart.