When Running Through the Airport,Collin Trips and Spills All of His

When running through the airport,Collin trips and spills all of his stuff out of his backpack.Lots of people see Collin trip and fall,but no one helps Collin pick up any of his stuff or check to see if he is okay.
A) Of what psychological concept is this an example?
B) Give one reason why this occurs.
C) Give one way you can reduce the possibility of this occurring.