What Is a "Lockout" in Connection with Labor Law

What is a "lockout" in connection with labor law?

A) an action by workers to prevent management from entering the company's premises
B) an action by workers to prevent customers from entering the company's premises
C) an action by workers to prevent suppliers from entering the company's premises
D) an action by management to prevent union organizers from entering the company's premises
E) an action by management to prevent workers from entering the company's premises