Retailers in a city discovered that some products they bought for retail sale were being sold by ...

Retailers in a city discovered that some products they bought for retail sale were being sold by distributors directly to the public, thereby reducing sales that could have gone to the retailers. The retailers got together and agreed not to buy any goods from distributors who sold directly to the public. This agreement is:
 a. an illegal boycott
  b. a legal boycott protected by right to free speech
  c. a legal boycott unless it has major economic consequences d. a violation of the Robinson-Patman Act
  e. none of the other choices