Why do pilots need a greater angle of attack at high altitude?

[gigglin_horse]

Homework Statement

"When a jet plane is cruising at high altitude, the flight attendants have more of a hill to climb as they walk forward along the aisle when the plane is cruising at low altitude. Why does the pilot have to fly with a great angle of attack at high altitude than at low altitude?"

Homework Equations

The Attempt at a Solution

Something to do with pressure?

 

[davieddy]

Density