What is the best way to undermine the social engineering activity of tailgating?

Bob waits near a secured door, holding a box. He waits until an employee walks up to the secured door and uses the special card in order to access the restricted area of the target company. Just as the employee opens the door, Bob walks up to the employee (still holding the box) and asks the employee to hold the door open so that he can enter.

What is the best way to undermine the social engineering activity of tailgating?
A . Issue special cards to access secure doors at the company and provide a one-time only brief description of use of the special card
B . Educate and enforce physical security policies of the company to all the employees on a regular basis
C . Setup a mock video camera next to the special card reader adjacent to the secure door
D . Post a sign that states, "no tailgating" next to the special card reader adjacent to the secure door

Answer: B