A defense available to the employer in an action for workers' compensation is that:

A) the employee assumed the risk.
B) the employee's injury happened before he came to work.
C) the employee was contributorily negligent.
D) a co-worker caused the injury.

The correct defense is that the employee's injury happened before he came to work. Workers' compensation is designed to cover injuries that occur in the course and scope of employment. If the injury occurred before the employee came to work, it would not be covered under workers' compensation. Assumption of risk, contributory negligence, and injuries caused by co-workers are generally not defenses in workers' compensation cases, as the system is designed to provide no-fault coverage for work-related injuries.