When Is It Legal for an Employer to Engage in an Employer

When is it legal for an employer to engage in an employer lockout?

A) at any time, so long as the purpose of the lockout is not malicious
B) any time there is at least thirty days advance notice
C) whenever one or more employees have violated the terms of a valid union employment contract
D) when the employer reasonably anticipates a strike by some of its employees
E) only after a strike has begun