When, if ever, is price discrimination allowed?

When, if ever, is price discrimination allowed?

A. If the seller can substantiate that it is distributing internationally, then it is legal.

B. If the seller can prove that its costs are different when selling to different retailers, then it is legal.

C. If the seller can prove that its revenue is affected when selling to similar retailers, then it is legal.

D. If the seller is selling via the internet as its main channel, then it is legal.