Which of the Following Is Probably an Illegal Boycott

Which of the following is probably an illegal boycott?

A) retailers join together to refuse to buy from distributors who do something the retailers do not like
B) distributors join together to refuse to sell to retailers who do something the wholesalers do not like
C) doctors in a city, agree through the City Medical Association, to refuse to take welfare patients because the city will not pay as much as the doctors think they should be paid
D) none of the other choices
E) all of the other specific choices