What is this an example of?

Since 9/11, airport parking garages now keep cars further away from the terminal entrance. What is this an example of?

A.
An administrative control

B.
A technical control

C.
An environmental control

D.
A physical control

Explanation:
Bollards are often used to protect buildings from traffic or simply as a deterrent from anything getting too close to the building. Since 9/11, many airports have putbollards in the parking spaces that used to possess cars and trucks. Now, a portion of the lots close to the terminal are left vacant to protect the building and peoplefrom potential terrorist attacks.