PurrFect Litter Clay Mines Employees Are Represented by a Union

PurrFect Litter Clay Mines employees are represented by a union. All of the employee mechanics who repair the company's equipment worked on the day shift, which meant that if the equipment broke down on one of the other shifts, PurrFect had to pay overtime or suffer lost production. PurrFect proposed to the union that the mechanics be divided among all three shifts to solve the problem but the union refused. Thereafter, PurrFect unilaterally changed the shift assignments in accordance with its proposals. The union filed an unfair labor practice charge and went on strike. PurrFect replaced the striking mechanics. Later, the NLRB found that PurrFect had committed an unfair labor practice. If the striking mechanics make an unconditional offer to return to work, PurrFect

A) must reinstate the mechanics because they were unfair labor practice strikers.
B) does not have to reinstate the mechanics because they were economic strikers.
C) does not have to reinstate the mechanics because they were sympathy strikers.
D) must reinstate the mechanics because they were wildcat strikers.