Under a Contract, Oil Shale Corporation Forbids Petro Inc

Under a contract, Oil Shale Corporation forbids Petro Inc., a wholesale buyer of Oil Shale's products, to purchase products from the seller's competitors. This is prohibited

A) under any circumstances.
B) if its effect is to stabilize the relevant market.
C) if its effect is to substantially lessen competition.
D) if tis purpose is to create a monopoly.