Mr Shades Sunglasses Promotes Its 'Buy One, Get One Half

Mr. Shades Sunglasses promotes its 'buy one, get one half price' offer in which customers who purchase one pair of sunglasses can purchase a second pair at half price. This is an example of _______and is _______.

A) conditional sales; only illegal if the practice substantially lessens competition or tends to create a monopoly
B) conditional sales; always illegal per se
C) price discrimination; always illegal per se
D) price discrimination; only illegal if the practice substantially lessens competition or tends to create a monopoly