What is a $$yellow$-$dog contract$?($p$.461)$

a$.$ It is a type of collective agreement that is highly favourable to the union, so called because the union is said to have struck fear into the heart of the employer with its threat of labour action

b$.$ It is a type of contract, now illegal, that requires an employee to promise to refrain from joining a union or to participate in any of its activities

c$.$ It is a type of contract that requires an employee to promise not to engage in a strike, but that they are otherwise free to belong to the union and participate in any of the union$$s other activities, such as collective bargaining